Hypercalcemia is an elevated serum calcium level above the normal range, most commonly caused by primary hyperparathyroidism or malignancy, potentially leading to renal, gastrointestinal, neuromuscular, and cardiac dysfunction.

Exam domain	One-glance essentials
Definition / diagnosis	Hypercalcaemia = raised adjusted total calcium or ionised calcium above the laboratory range; confirm an unexpected result. *Adjusted Ca²⁺ > 3.0 mmol/L usually warrants urgent treatment; > 3.5 mmol/L is a medical emergency* ^[1].
Epidemiology / risks	*Primary hyperparathyroidism and malignancy account for most cases* ^[1] Other causes: thiazides, lithium, vitamin D/A excess, granulomatous disease, thyrotoxicosis, immobilisation, and familial hypocalciuric hypercalcaemia (FHH).
Core mechanism	First split by PTH: raised/inappropriately normal PTH = PTH-dependent; suppressed PTH = malignancy or another PTH-independent cause. Hypercalcaemia causes nephrogenic DI → polyuria/dehydration → reduced GFR, which further worsens calcium.
Clinical picture	Stones, bones, abdominal groans, and psychic moans: nephrolithiasis, bone pain, constipation/abdominal pain, weakness, confusion/depression. Severe disease: *dehydration, delirium/reduced consciousness, shortened QT, and arrhythmia* ^[1]^[2].
Investigations	Repeat adjusted and/or ionised Ca²⁺, then check PTH first; also phosphate, Mg²⁺, creatinine/eGFR, ECG, and medication history. If PTH suppressed: investigate malignancy (± PTHrP, myeloma screen), vitamin D excess/granulomatous disease, and thyroid disease as indicated. If PTH non-suppressed: distinguish PHPT from FHH using urine calcium/clearance ratio before surgery.
Management	*Stop contributors, restore euvolaemia with IV 0.9% saline, then give antiresorptive therapy for severe/symptomatic disease* ^[1] Calcitonin acts rapidly but briefly; IV bisphosphonate has delayed, sustained action—choose/adjust antiresorptive therapy for renal function and cause. Treat the cause; dialysis is reserved for life-threatening/refractory hypercalcaemia or when fluids/drugs are unsafe.
Complications	AKI, nephrolithiasis/nephrocalcinosis, pancreatitis, peptic symptoms, osteoporosis/fracture, neuropsychiatric deterioration, and arrhythmia/coma.
Prevention / follow-up	Review calcium/vitamin D supplements and culprit drugs; maintain hydration and mobility in at-risk patients. Monitor calcium and renal function in known PHPT/malignancy and after antiresorptive treatment.
Exam traps	Do not interpret total calcium without albumin context; use ionised calcium when albumin/pH makes correction unreliable ^[3]. Never infer FHH from PTH alone—urine calcium is the discriminator. Routine loop diuretics do not treat hypercalcaemia and may worsen dehydration; reserve them for fluid overload after rehydration.

References

^[1] Lecture slides: GC 039. Parathyroid and Calcium Disorders.pdf (pp10, 15, 23, 28 — causes, severity, emergency management, complications) ^[2] Senior notes: Confused Patients – A case of hypercalcemia.pdf (pp1–2 — dehydration and acute neurocognitive presentation) ^[3] Lecture slides: Data Interpretation Workshop - Chemical Pathology.pdf (pp38, 40 — albumin adjustment and ionised calcium)

Hypercalcemia

Hypercalcemia literally means "hyper" (excessive) + "calc" (calcium) + "emia" (in the blood). It is defined as an elevated serum corrected calcium > 2.55 mmol/L (or ionized calcium above the upper limit of normal) ^[1]^[2].

Before interpreting any calcium result, you must correct for albumin, because ~40% of circulating calcium is protein-bound (mainly albumin). If albumin is low (e.g. in cirrhosis, nephrotic syndrome, malnutrition), total calcium will be falsely low even though ionized (biologically active) calcium is normal. Conversely, dehydration with haemoconcentration can raise total calcium artefactually.

Corrected Ca²⁺ (mmol/L) = Total Ca + [0.02 × (40 − albumin in g/L)] ^[1]^[2]^[3]

Why do we correct? 50% of plasma calcium is ionized (the physiologically active fraction), 40% is albumin-bound (inactive), and 10% is anion-bound (citrate, phosphate). Only the ionized fraction matters physiologically. When albumin drops, total calcium drops but ionized calcium stays the same — the patient is normocalcaemic. The correction formula estimates what total calcium would be if albumin were normal (40 g/L).

When the correction formula breaks down

The correction formula does NOT apply when albumin < 20 g/L or in the presence of multiple myeloma or dysglobulinaemia (paraproteins bind calcium unpredictably). In these situations, request ionized calcium directly from the lab ^[1]^[2].

2. Epidemiology and Risk Factors

Hypercalcemia is common: prevalence in the general population is approximately 1–3%.
The two most common causes account for ~90% of all hypercalcemia: primary hyperparathyroidism (PHPT) and malignancy ^[2]^[4].
- Outpatient (ambulatory) setting: PHPT is the #1 cause (often an incidental finding on routine bloods — "comes with their insurance packages" ^[4]).
- Inpatient (hospital) setting: Malignancy-associated hypercalcemia (MAH) is the #1 cause.
PHPT: Incidence 1–2 per 1000; peaks in 6th–7th decade; female preponderance (F:M ≈ 2–3:1), particularly postmenopausal women ^[4].
Malignancy-associated hypercalcemia occurs in 10–30% of all cancer patients at some point during disease, and is a poor prognostic sign (median survival ~30 days if untreated).

Risk Factor	Mechanism
Postmenopausal female	Oestrogen withdrawal → ↑ bone resorption; also ↑ incidence of parathyroid adenoma
Malignancy (lung SCC, breast, renal, myeloma, lymphoma)	PTHrP secretion, osteolytic metastases, 1,25(OH)₂D production (lymphoma)
Lithium use	Shifts the calcium-PTH set point rightward → PTH secreted at higher calcium levels ^[5]
Thiazide diuretics	↑ Renal calcium reabsorption at the DCT (unlike loop diuretics which are calciuric) ^[6]
Vitamin D supplementation / granulomatous disease	Excessive 1,25(OH)₂D
Immobilization	↑ Osteoclastic bone resorption
Familial (MEN syndromes, FHH)	Genetic predisposition
Chronic kidney disease (tertiary hyperparathyroidism)	Autonomous PTH secretion after prolonged secondary stimulation

3. Anatomy and Physiology: Calcium Homeostasis

Total body calcium: 99% in bone (as hydroxyapatite crystals), 0.3% in muscle, 0.1% in extracellular fluid ^[1].
Plasma calcium forms: ionized Ca²⁺ (50%) — the biologically active fraction; protein-bound (40%) — mainly albumin and globulin; anion-bound (10%) — citrate and phosphate ^[1].

3.2 The Three Hormones of Calcium Homeostasis

Three hormones regulate calcium: PTH, Vitamin D (calcitriol), and Calcitonin ^[1]^[7].

Secreted by chief cells of the parathyroid glands ^[7].
Half-life: ~4 minutes (very short — allows rapid minute-to-minute regulation) ^[7].
Actions (all raise serum calcium):
1. Bone: Stimulates osteoclasts (indirectly via osteoblast RANKL signalling) → releases calcium and phosphate from bone.
2. Kidney:
  - ↑ Ca²⁺ reabsorption at the distal convoluted tubule.
  - ↓ phosphate reabsorption at the proximal tubule (phosphaturic) — this is crucial because if phosphate also rose with calcium, calcium-phosphate product would increase → ectopic calcification.
  - Activates vitamin D by stimulating 1α-hydroxylase in the kidney (converts 25(OH)D → 1,25(OH)₂D).
3. Gut (indirect): Via vitamin D activation → ↑ intestinal calcium and phosphate absorption.

PTH responds more to phosphate level than calcium levels — this is why secondary hyperparathyroidism is the mechanism of hypocalcemia in renal disease (phosphate retention → stimulates PTH) ^[3].

Mechanism of vitamin D production requires hydroxylation in both liver and kidney ^[1]:

Site	Enzyme/Mediator	Reaction
Skin	UV light	7-dehydrocholesterol → Previtamin D → Cholecalciferol (Vitamin D₃)
Liver	Vitamin D 25-hydroxylase	Cholecalciferol → Calcidiol (25-hydroxyvitamin D)
Kidney	1α-hydroxylase	Calcidiol → Calcitriol (1,25-dihydroxyvitamin D)

25(OH)D (calcidiol) is the storage form and best reflects vitamin D status.
1,25(OH)₂D (calcitriol) is the active form but has a short half-life and is tightly regulated.
Actions: ↑ intestinal calcium and phosphate absorption; works with PTH on bone resorption; ↑ renal calcium reabsorption.

Secreted by C cells (parafollicular cells) of the thyroid gland ^[7].
Half-life: ~60 minutes ^[7].
Antagonizes PTH action: inhibits osteoclast activity → ↓ bone resorption → ↓ serum calcium ^[7].
Physiological role in humans is modest (thyroidectomy patients don't get hypercalcemia from calcitonin deficiency), but pharmacological doses are used therapeutically.

The calcium-sensing receptor (CaSR) on parathyroid chief cells is the key sensor. When ionized calcium is high, CaSR is activated → suppresses PTH release. This receptor is critical for understanding:

Familial Hypocalciuric Hypercalcemia (FHH): Loss-of-function mutation of CaSR → parathyroid gland "thinks" calcium is lower than it actually is → continues secreting PTH despite mild hypercalcemia → benign condition requiring NO surgery.
Calcimimetics (e.g. cinacalcet): Allosteric activators of CaSR → make the receptor more sensitive to calcium → ↓ PTH secretion. Used in secondary/tertiary hyperparathyroidism and parathyroid carcinoma.

4. Etiology

90% Rule

Hypercalcemia: most common (90%) is Malignancy + Primary Hyperparathyroidism ^[2]. So when you see incidental hypercalcemia, your two main differentials are PHPT and malignancy. Always exclude malignancy in any new presentation.

4.1 PTH-Mediated Hypercalcemia (PTH is HIGH or inappropriately normal)

Most common cause of hypercalcemia overall (especially outpatient) ^[4].
Causes of PHPT ^[4]:
- Solitary parathyroid adenoma (85%)
- Parathyroid hyperplasia (10–15%)
- Double adenomas (1–2%)
- Parathyroid carcinoma (~1%)
Associated with MEN syndromes:
- MEN 1 (3Ps: Parathyroid, Pituitary, Pancreatic tumours) — most common component is PHPT (~95%).
- MEN 2A (Medullary thyroid cancer, Phaeochromocytoma, Parathyroid hyperplasia).

Occurs after prolonged secondary hyperparathyroidism (usually in CKD) → parathyroid glands become autonomous and no longer respond to calcium feedback.
PTH is very high, calcium is high, and phosphate may be high (unlike PHPT where phosphate is low).

Lithium causes hyperparathyroidism / hypercalcemia ^[5].
Mechanism: Lithium shifts the CaSR set point → PTH is secreted at higher calcium concentrations. Can also cause parathyroid hyperplasia with chronic use.

Must exclude FHH for hypercalcemia → not due to parathyroid, so don't unnecessarily refer these patients to surgery ^[2].
Autosomal dominant loss-of-function mutation of CaSR.
Features: Mild hypercalcemia, low urinary calcium excretion (calcium:creatinine clearance ratio < 0.01), normal or mildly elevated PTH.
Benign — requires no treatment. Important to differentiate from PHPT (which needs surgery).
Diagnosis: 24-hour urine calcium and calcium:creatinine clearance ratio ^[2].

4.2 Non-PTH-Mediated Hypercalcemia (PTH is appropriately SUPPRESSED / low)

The most common cause in hospitalized patients. Four mechanisms:

Mechanism	Tumour Type	Pathophysiology
Humoral hypercalcemia of malignancy (HHM) — PTHrP	Squamous cell carcinomas (lung, head/neck, oesophagus), renal cell carcinoma, breast, bladder	Tumour secretes PTHrP (parathyroid hormone-related peptide) which mimics PTH action on bone and kidney (~80% of MAH)
Osteolytic metastases	Breast cancer, multiple myeloma, lung cancer	Direct bone destruction by tumour → local release of calcium. In myeloma, myeloma cells secrete RANKL, MIP-1α, and other osteoclast-activating factors
Ectopic 1,25(OH)₂D production	Lymphoma (Hodgkin's and NHL)	Tumour macrophages express 1α-hydroxylase → excess calcitriol production
Ectopic PTH secretion	Rare (some ovarian, lung tumours)	Tumour produces true PTH (extremely rare)

Multiple Myeloma and ALP

In myeloma, despite all the bony involvement, ALP will be normal. High ALP indicates active bone mineralization and formation, but myeloma causes purely lytic lesions with no massive compensatory increase in bone formation ^[8]. This is a classic exam trap — if you see lytic bone lesions + normal ALP, think myeloma. Metastatic bone disease (especially prostate, breast) typically has elevated ALP because of mixed lytic/blastic or blastic activity.

Paraneoplastic hypercalcemia (via PTHrP) is one of the most important paraneoplastic syndromes to know. Squamous cell carcinoma of the lung is a classic culprit ^[9]^[10].

Sarcoidosis, tuberculosis (relevant in Hong Kong where TB prevalence is higher than Western countries), fungal infections, berylliosis.
Mechanism: Activated macrophages within granulomas express 1α-hydroxylase → autonomous production of 1,25(OH)₂D → ↑ calcium absorption from gut and ↑ bone resorption.
This is vitamin D–mediated and responds to glucocorticoids (which suppress macrophage 1α-hydroxylase activity).

Drug	Mechanism
Thiazide diuretics	↑ Calcium reabsorption at DCT (enhance Na⁺/Ca²⁺ exchange) ^[6]. NB: Don't answer loop diuretics as treatment for hypercalcemia since the calciuric effect is not that marked ^[6]
Vitamin D / Calcium supplements (over-supplementation)	Direct increase in calcium absorption
Vitamin A toxicity	Stimulates osteoclast activity
Milk-alkali syndrome (excessive antacids + milk)	↑ Calcium intake + ↑ renal calcium reabsorption due to alkalosis

Thyrotoxicosis: Thyroid hormones stimulate osteoclast activity → ↑ bone turnover → mild hypercalcemia (usually < 3.0).
Adrenal insufficiency (Addison's disease): Cortisol normally inhibits intestinal calcium absorption and promotes renal calcium excretion; in deficiency, calcium rises. Also haemoconcentration from volume depletion.
Phaeochromocytoma: Can co-occur with PHPT in MEN 2A.
Acromegaly: GH/IGF-1 stimulate renal 1α-hydroxylase.

Prolonged bed rest (especially in Paget's disease, young patients with fractures) → osteoclastic resorption exceeds formation → calcium release from bone.
Mechanism: Mechanical loading normally stimulates osteoblasts; in its absence, osteoclastic activity predominates.

Hypercalcemia is an etiology of chronic pancreatitis ^[11].
CPPD (pseudogout) is associated with hyperparathyroidism (3.35× risk) ^[12].

In Hong Kong, key aetiologies to keep in mind:

PHPT — most common cause overall, often incidental finding on health screening (very common in HK where annual check-ups are routine).
Malignancy — lung cancer (high prevalence in HK due to smoking and environmental factors), hepatocellular carcinoma (HBV-related), nasopharyngeal carcinoma (endemic in Southern China), multiple myeloma.
Granulomatous disease — TB is still prevalent in HK.
Drug-related — thiazides (commonly prescribed for hypertension), lithium (psychiatric patients), calcium/vitamin D over-supplementation (osteoporosis treatment in ageing population).

5. Classification

Three stages of hypercalcemia severity ^[4]:

Severity	Serum Calcium	Clinical Implication
Asymptomatic / Mildly Symptomatic	< 3.0 mmol/L	Most patients present at this stage (incidental finding)
Moderate	3.0–3.5 mmol/L	Symptomatic; requires active management
Severe	> 3.5 mmol/L	Hypercalcemic crisis — medical emergency, risk of cardiac arrhythmias, coma, death

Category	PTH Level	Key Causes
PTH-mediated	High / inappropriately normal	PHPT, tertiary HPT, lithium, FHH
Non-PTH-mediated	Suppressed (low)	Malignancy (PTHrP, osteolytic), granulomatous disease, vitamin D excess, drugs, immobilization, thyrotoxicosis

Acute hypercalcemia → typically malignancy, usually severe, rapid onset.
Chronic hypercalcemia → typically PHPT, often mild, long-standing.

The final common pathways leading to hypercalcemia are:

↑ Bone resorption (most common mechanism) — PTH, PTHrP, osteolytic metastases, immobilization, thyrotoxicosis.
↑ Intestinal calcium absorption — Vitamin D excess, granulomatous disease (ectopic 1,25(OH)₂D).
↓ Renal calcium excretion — Thiazides, FHH, dehydration (reduced GFR → less calcium filtered).
↑ Calcium intake — Milk-alkali syndrome, excessive supplementation (rarely sufficient alone without impaired excretion).

In most cases, a vicious cycle ensues:

Hypercalcemia → polyuria (nephrogenic diabetes insipidus mechanism: calcium deposits in renal medulla and antagonizes ADH at the collecting duct) → dehydration.
Dehydration → ↓ GFR → ↓ renal calcium excretion → worsening hypercalcemia.
Worsening hypercalcemia → more confusion, nausea, vomiting → further dehydration.

This is why the lecture title is "Confused and dehydrated: hypercalcemia" ^[1]^[13] — confusion and dehydration are the hallmark presentation of significant hypercalcemia.

7. Clinical Features

This mnemonic covers the major organ systems affected. While originally described for primary hyperparathyroidism, it applies to hypercalcemia from any cause. These classical features are rare now because treatments are getting better and patients are not presenting as late ^[4].

7.1 Symptoms (with Pathophysiological Basis)

Symptom	Pathophysiology
Bone pain	↑ Osteoclast activity → bone resorption → subperiosteal erosions, microfractures
Fractures (pathological)	Weakened bone structure from excessive resorption (osteoporosis/osteopenia)
Joint pain	Calcification of cartilage → chondrocalcinosis / pseudogout (CPPD deposition) ^[4]

Symptom	Pathophysiology
Renal colic / kidney stones	Hypercalciuria → calcium oxalate/phosphate stone formation in renal collecting system
Nephrocalcinosis	Diffuse calcium deposition within renal parenchyma
Polyuria and polydipsia	Calcium interferes with ADH action at the collecting duct (acquired nephrogenic DI) + also activates CaSR in the thick ascending limb → inhibits Na-K-2Cl cotransporter (NKCC2) → impaired concentrating ability
Renal failure	Nephrocalcinosis → tubulointerstitial damage; also prerenal from dehydration

Symptom	Pathophysiology
Anorexia, nausea, vomiting	Calcium stimulates gastrin secretion → ↑ gastric acid; also direct effect on CTZ and GI smooth muscle
Constipation	Hypercalcemia → ↓ smooth muscle contractility in the GI tract (calcium stabilizes cell membranes → ↓ excitability, paradoxically reducing peristalsis despite intracellular calcium's role in contraction — the effect here is on the extracellular side of the membrane)
Epigastric pain / dyspepsia	↑ Gastrin secretion → peptic ulcer disease
Pancreatitis (acute)	Calcium activates trypsinogen → trypsin within the pancreatic duct → autodigestion. Also, calcium deposits in the pancreatic duct
Ileus	Reduced GI motility from smooth muscle hypotonia
Thirst, dry mouth	Dehydration from polyuria and vomiting

Symptom	Pathophysiology
Fatigue / tiredness	Neuromuscular effects of hypercalcemia (↓ neuromuscular excitability)
Weakness (proximal myopathy)	Calcium stabilizes membranes → ↓ excitability of muscle fibres → weakness
Depressed mood	Central neurological effect of calcium on neurotransmitter release
Mental confusion and drowsiness	High calcium → ↓ neuronal excitability → altered consciousness
Psychosis	Severe hypercalcemia can cause frank psychotic symptoms
Coma (in severe cases)	Extreme membrane stabilization → diffuse neuronal depression

"Hypercalcemia most alarming → confused, dehydrated, risk of cardiac arrhythmias" ^[8]

Symptom	Pathophysiology
Hypertension	Calcium → vasoconstriction of vascular smooth muscle; also volume contraction from dehydration activates RAAS
Palpitations / arrhythmias	Shortened QT interval → risk of ventricular arrhythmias; also ↑ sensitivity to digoxin

Memory impairment (cognitive decline) ^[7]
Band keratopathy (calcium deposition in the cornea — visible on slit-lamp examination)

Sign	Pathophysiology / Significance
Dehydration (dry mucous membranes, reduced skin turgor, tachycardia, postural hypotension)	Polyuria + vomiting + poor oral intake → volume depletion
Confusion / altered mental state	CNS depression from hypercalcemia
Proximal muscle weakness	Membrane stabilization → ↓ neuromuscular excitability
Hyporeflexia	Same mechanism as weakness — ↓ neuronal excitability
Abdominal distension / reduced bowel sounds	Ileus from smooth muscle hypotonia
Short QT interval on ECG	↑ Extracellular calcium → faster phase 2 repolarization (calcium accelerates the plateau phase of the cardiac action potential) → shortened ST segment → shortened QT
Band keratopathy	Metastatic calcification in cornea (chronic hypercalcemia)
Neck mass (if parathyroid adenoma is large enough — rare)	Direct palpation of adenoma
"Salt-and-pepper" degranulation of skull	Subperiosteal bone resorption on skull X-ray (severe hyperparathyroidism) ^[7]
Brown tumours (osteitis fibrosa cystica)	Collections of osteoclasts, fibrous tissue, and haemosiderin in bone (severe long-standing PHPT)

ECG findings (contrast with hypocalcemia which causes prolonged QT):

Shortened QT interval (most characteristic finding)
Shortened ST segment (the ST segment may be virtually absent)
Widened T wave
In severe cases: Osborn (J) waves, bradycardia, heart block, and ultimately cardiac arrest
↑ Sensitivity to digitalis → risk of digoxin toxicity → avoid digoxin in hypercalcemia

Calcium and the QT Interval — Memory Aid

Think of calcium as a "membrane stabilizer" — it shortens the action potential plateau (phase 2). So:

Hypercalcemia → short QT
Hypocalcemia → long QT (more time for calcium to enter → prolonged plateau)

This is the opposite of potassium's effect on the QRS/T wave morphology, so keep them distinct.

It is important to also look for features of the cause:

Aetiology	Clinical Clues
PHPT	Postmenopausal woman, renal stones, osteoporosis, family history (MEN)
Malignancy	Weight loss, lymphadenopathy, hepatomegaly, bone pain, smoking history, known cancer
Multiple myeloma	CRAB: Calcium elevation, Renal failure, Anaemia, Bone lesions (lytic) ^[8]; Bence Jones proteinuria, high globulin, rouleaux formation
Sarcoidosis	Bilateral hilar lymphadenopathy on CXR, erythema nodosum, uveitis, skin lesions
Thyrotoxicosis	Tremor, weight loss, tachycardia, exophthalmos, goitre
Drug history	Thiazides, lithium, vitamin D/calcium supplements, vitamin A
FHH	Family history of mild hypercalcemia, asymptomatic, no renal stones

High Yield Summary

Definition: Corrected Ca²⁺ > 2.55 mmol/L. Always correct for albumin: Corrected Ca = Total Ca + 0.02 × (40 − albumin g/L). Correction fails when albumin < 20 or paraproteinaemia → measure ionized Ca directly.

90% rule: PHPT + Malignancy account for ~90% of all hypercalcemia.

PHPT: #1 cause overall; 85% solitary adenoma; peaks 6th–7th decade; F > M (postmenopausal).

Malignancy: #1 cause in inpatients; PTHrP (SCC lung), osteolytic (myeloma, breast), ectopic 1,25(OH)₂D (lymphoma). In myeloma, ALP is characteristically NORMAL despite lytic lesions.

FHH: Must exclude before referring for parathyroidectomy — use 24h urine Ca and Ca:Cr clearance ratio < 0.01.

Severity: Mild < 3.0; Moderate 3.0–3.5; Severe > 3.5 mmol/L.

Clinical features: "Bones, Stones, Abdominal Groans, Psychiatric Overtones" — now rare due to earlier detection.

ECG: Short QT (calcium shortens action potential plateau). Contrast with hypocalcemia → prolonged QT.

Vicious cycle: Hypercalcemia → polyuria → dehydration → ↓ GFR → ↓ Ca excretion → worse hypercalcemia.

Key drugs: Thiazides (↑ Ca reabsorption, unlike loops). Lithium (shifts CaSR set point). Don't use loop diuretics as primary treatment for hypercalcemia — effect is not that marked.

Calcium homeostasis: PTH (4 min half-life, chief cells) vs Calcitonin (60 min, C cells). PTH raises Ca; calcitonin lowers it. Vitamin D needs liver (25-hydroxylase) and kidney (1α-hydroxylase) for activation.

Active Recall - Hypercalcemia (Definition to Clinical Features)

^[1] Lecture slides: MBBS Final MB (Medicine) (Felix PY Lai).pdf — Hypercalcemia section ^[2] Senior notes: Chemical Pathology Data interpretation.pdf ^[3] Senior notes: Endocrine Interactive Tutorial.pdf ^[4] Senior notes: Block A - Confused and dehydrated: hypercalcaemia; hypocalcaemia.pdf ^[5] Senior notes: Block A - Drugs and the Kidney.pdf — Lithium section ^[6] Senior notes: Block A - Clinical Pharmacology of anti-HT and anti-HF medications.pdf — Thiazide diuretics ^[7] Senior notes: Maksim Surgery Notes.pdf — Parathyroid section ^[8] Senior notes: Block A - An old man with bone pain and anaemia: multiple myeloma; monoclonal gammopathy.pdf ^[9] Lecture slides: Clinical manifestation of lung cancer (1).pdf ^[10] Lecture slides: Paraneoplastic Syndrome_rev1 (1).pdf ^[11] Senior notes: Block A - Upper abdominal pain: peptic ulcer; pancreatitis and gallstone.pdf ^[12] Senior notes: Ryan Ho Rheumatology.pdf — CPPD section ^[13] Lecture slides: GC 039. Confused and dehydrated: hypercalcaemia; hypocalcaemia.pdf

Differential Diagnosis of Hypercalcemia

When you find a corrected calcium > 2.55 mmol/L, the very first question you need to answer is: "Is the PTH appropriately suppressed, or is it elevated/inappropriately normal?" This single branch point divides the entire differential into two clean categories and dictates every subsequent investigation.

Hypercalcemia, most common (90%) is Malignancy + Primary hyperparathyroidism ^[2]. So when you encounter hypercalcemia, these two diagnoses must sit at the top of your differential list at all times.

The logic is simple from first principles:

Calcium is high → the parathyroid gland should sense this via the calcium-sensing receptor (CaSR) → PTH should be suppressed.
If PTH is high or inappropriately normal despite hypercalcemia, something is wrong with the parathyroid axis itself (autonomous PTH secretion, abnormal CaSR set point, or drug interference).
If PTH is appropriately suppressed (low), the calcium is coming from somewhere else — the cancer is making PTHrP, bone is being destroyed directly, vitamin D is in excess, or there is excess intake.

3. Detailed Differential Diagnosis Table

Diagnosis	Key Features	Distinguishing Investigations	Pathophysiology
Primary Hyperparathyroidism (PHPT)	Most common cause of hypercalcemia overall; peaks 6th–7th decade; F > M; often asymptomatic/incidental ^[4]; bones/stones/abdominal groans/psychiatric overtones	↑ PTH, ↑ Ca, ↓ PO₄, ↑ urinary Ca (Ca:Cr clearance ratio > 0.02); ALP may be normal or elevated (elevated if significant osteitis fibrosa cystica); imaging: sestamibi scan, neck US	Solitary adenoma (85%), hyperplasia (10–15%), double adenoma (1–2%), carcinoma (~1%) ^[4]; autonomous PTH secretion → ↑ bone resorption, ↑ renal Ca reabsorption, ↑ 1α-hydroxylase → ↑ gut absorption
Tertiary Hyperparathyroidism	History of CKD (usually on dialysis); very high PTH; calcium and phosphate both often high	↑↑ PTH (much higher than PHPT), ↑ Ca, ↑ PO₄ (unlike PHPT where PO₄ is low); renal osteodystrophy on imaging	Prolonged secondary hyperparathyroidism → parathyroid gland hyperplasia becomes autonomous and no longer responds to calcium/calcitriol feedback ^[14]
Lithium-induced	Psychiatric history; on lithium for bipolar disorder	↑ PTH (or inappropriately normal), ↑ Ca, often normal PO₄; resolves on cessation (sometimes)	Lithium shifts the CaSR set point rightward → PTH secreted at higher Ca concentrations; can also cause parathyroid hyperplasia ^[5]
Familial Hypocalciuric Hypercalcemia (FHH)	Family history of mild asymptomatic hypercalcemia; no renal stones; benign lifelong condition	Ca:Cr clearance ratio < 0.01; 24-hour urine calcium is LOW (< 100 mg/day); PTH normal or mildly elevated; genetic testing for CaSR mutation	AD loss-of-function mutation of CaSR → parathyroid "thinks" calcium is low → PTH not appropriately suppressed; kidneys also reabsorb more calcium (CaSR also on renal tubules)

PHPT vs FHH — The Critical Distinction

Remember to exclude FHH for hypercalcemia → not due to parathyroid, so don't unnecessarily refer these patients to surgery ^[2]. Both have elevated/normal PTH with hypercalcemia. The differentiator is the 24-hour urine calcium and calcium:creatinine clearance ratio:

PHPT: High urinary calcium (ratio > 0.02) — the kidneys are filtering excess calcium
FHH: Low urinary calcium (ratio < 0.01) — the kidneys avidly reabsorb calcium due to CaSR defect

FHH is benign and requires no intervention. Sending a patient with FHH for parathyroidectomy is both unnecessary and unlikely to cure the hypercalcemia (because the problem is systemic CaSR dysfunction, not the parathyroid glands per se). Have to do 24-hour urine, exclude FHH ^[2].

Phosphate is a key discriminator: in PHPT, phosphate should be LOW (PTH causes renal phosphate wasting). If phosphate is normal in a patient you suspect has PHPT, you must question the diagnosis and exclude FHH ^[2].

Diagnosis	Key Features	Distinguishing Investigations	Pathophysiology
Humoral Hypercalcemia of Malignancy (HHM)	Weight loss, constitutional symptoms; Lung SCC, renal cell CA, breast, bladder, ovarian ^[1]	↑ PTHrP; ↓ PTH; ↓ PO₄ (PTHrP mimics PTH phosphaturic effect); hypercalcemia often severe (> 3.5)	Tumour secretes PTHrP → acts on same PTH/PTHrP receptor on bone and kidney → ↑ bone resorption, ↑ renal Ca reabsorption. Accounts for ~80% of malignant hypercalcemia
Osteolytic Metastases	Known primary malignancy (breast, lung); Multiple Myeloma (CRAB: Calcium, Renal failure, Anaemia, Bone lesions) ^[8]	Imaging: lytic lesions on skeletal survey/CT; myeloma: normal ALP ^[8], ↑ globulin, paraprotein on SPEP/UPEP, Bence Jones protein; breast mets: ↑ ALP (mixed lytic/blastic)	Direct bone destruction releases calcium locally; myeloma cells produce RANKL, MIP-1α, DKK1 → osteoclast activation + osteoblast suppression
Ectopic 1,25(OH)₂D production	Lymphoma (Hodgkin's/NHL); granulomatous diseases (see below)	↑ 1,25(OH)₂D; ↓ PTH; ↓ PTHrP; ↑ urinary Ca	Tumour/granuloma macrophages express 1α-hydroxylase autonomously (not regulated by PTH or Ca) → excess calcitriol → ↑ gut Ca absorption
Ectopic PTH secretion	Extremely rare; lung, ovarian, pancreatic, thyroid papillary, neuroectodermal, rhabdomyosarcoma ^[1]	↑ intact PTH from tumour (confirmed by tumour tissue PTH immunohistochemistry); PTHrP negative	Tumour produces biologically active PTH — distinguished from PTHrP by specific assays
Granulomatous Disease (Sarcoidosis, TB, fungal)	Sarcoidosis: bilateral hilar LAD, erythema nodosum, uveitis; TB: endemic in Hong Kong	↑ 1,25(OH)₂D; ↑ ACE level (sarcoidosis); ↓ PTH; may have ↑ urinary Ca	Macrophages in granulomas express 1α-hydroxylase → autonomous calcitriol production (unregulated by negative feedback). Responds to glucocorticoids which suppress macrophage 1α-hydroxylase
Thyrotoxicosis	Tremor, weight loss, tachycardia, heat intolerance, goitre; occurs in 15–20% of thyrotoxic patients ^[1]	Suppressed TSH, ↑ FT4/FT3; ↓ PTH; usually mild hypercalcemia (< 3.0)	Thyroid hormones directly stimulate osteoclast activity → ↑ bone turnover/resorption. Calcium usually only mildly elevated
Vitamin D Toxicity	History of excessive supplementation (> 50,000 IU/day) or calcitriol use	↑ 25(OH)D (if cholecalciferol excess) or ↑ 1,25(OH)₂D (if calcitriol excess); ↓ PTH	Excess calcitriol → ↑↑ intestinal calcium absorption; ↑ bone resorption
Vitamin A Excess	History of supplement abuse, retinoid therapy (e.g. all-trans retinoic acid for APL) ^[1]	↑ Serum vitamin A level; ↓ PTH	Hypervitaminosis A (> 50,000 IU/day) → ↑ IL-6 concentration → stimulates osteoclast-mediated bone resorption ^[1]
Thiazide Diuretics	Drug history; usually only mild hypercalcemia	Drug history; ↓ urinary Ca; ↓ PTH (or normal)	Thiazides increase calcium reabsorption at the DCT — enhance the basolateral Na⁺/Ca²⁺ exchanger by depleting intracellular sodium via NCC blockade. Different to loop diuretics which cause calciuresis ^[6]
Immobilization	Prolonged bed rest; young patients with fractures; patients with high bone turnover (Paget's, malignancy)	↓ PTH; ↑ urinary Ca; ↑ bone resorption markers	Absence of mechanical loading → osteoclastic resorption predominates over formation → net calcium release from bone
Adrenal Insufficiency	Hypotension, hyperpigmentation (Addison's), hyponatraemia, hyperkalaemia	↓ Morning cortisol; ↓ ACTH (secondary) or ↑ ACTH (primary); short Synacthen test	Cortisol normally suppresses intestinal Ca absorption and promotes renal Ca excretion; loss of cortisol → ↑ absorption + ↓ excretion. Also haemoconcentration from volume depletion
Milk-Alkali Syndrome	History of excessive antacid/calcium ingestion; triad of hypercalcemia + metabolic alkalosis + renal impairment	↓ PTH; metabolic alkalosis on ABG; ↑ creatinine	Excess oral calcium intake + alkalosis → ↑ renal calcium reabsorption (alkalosis favours Ca binding to albumin in tubular fluid, reducing free Ca for excretion)

Understanding the clinical context drastically narrows the differential:

Setting	Most Likely Cause	Reasoning
Asymptomatic outpatient (incidental finding on health screening)	Primary hyperparathyroidism	"Since it comes with their insurance packages, incidental finding during the health check" ^[4]; mild, chronic, often < 3.0 mmol/L
Hospitalized / acutely unwell patient	Malignancy-associated hypercalcemia	Often severe (> 3.5), rapid onset, associated with weight loss and constitutional symptoms
CKD patient on dialysis	Tertiary hyperparathyroidism	Autonomous PTH after prolonged secondary HPT stimulation ^[14]
Psychiatric patient on lithium	Lithium-induced hyperparathyroidism	Lithium causes hyperparathyroidism / hypercalcemia ^[5]
Patient on thiazide diuretics for hypertension	Thiazide-related hypercalcemia	Hypercalcemia due to increased reabsorption ^[6]; often unmasks underlying mild PHPT
Young patient with lymphadenopathy, bilateral hilar LAD	Sarcoidosis or lymphoma	Ectopic 1,25(OH)₂D production
Post-thyroidectomy / parathyroidectomy	Hungry bone syndrome (actually causes hypocalcemia — important DDx exclusion)	Rapid bone uptake of calcium post-parathyroidectomy when chronically elevated PTH is suddenly removed

5. Distinguishing Key Mimics and Pitfalls

Before launching into an extensive workup, always rule out laboratory error and artefact:

Prolonged tourniquet application during phlebotomy → haemoconcentration → falsely elevated total calcium.
Dehydration → haemoconcentration → elevated total calcium (but ionized calcium may be normal).
Hyperalbuminaemia (rare, e.g. severe dehydration) → ↑ total calcium but ionized calcium is normal.
In multiple myeloma / dysglobulinaemia: paraproteins can bind calcium unpredictably → total calcium unreliable → measure ionized calcium directly ^[1].
EDTA contamination has to be excluded (EDTA chelates calcium → artefactually LOW calcium; relevant for the hypocalcemia DDx but important to know about lab interference in general) ^[2].

Feature	Primary Hyperparathyroidism	Malignancy
Onset	Chronic, insidious	Acute, rapid
Severity	Usually mild (< 3.0)	Often moderate–severe (> 3.0, frequently > 3.5)
PTH	↑ or inappropriately normal	↓ (suppressed)
PTHrP	Negative	Often positive (HHM)
Phosphate	↓ (PTH causes phosphaturia)	↓ (if PTHrP-mediated) or normal/↑ (osteolytic)
ALP	Normal or mildly ↑	Often ↑ (bone mets) or normal in myeloma ^[8]
1,25(OH)₂D	↑ (PTH activates 1α-hydroxylase)	Usually ↓ or normal (↑ only in lymphoma)
Chloride	Mild ↑ (PTH causes ↑ HCO₃⁻ excretion → hyperchloraemic acidosis)	Normal
Constitutional symptoms	Absent	Present (weight loss, fatigue, anorexia)
Urinary calcium	↑ (need to r/o FHH if < 0.01)	↑
Duration	Often years	Weeks to months

So incidental finding of hyperparathyroidism → have to exclude malignancy ^[2]. Even when PTH is elevated and you think you have PHPT, you should still screen for occult malignancy (age-appropriate cancer screening, basic bloods including SPEP, CXR) because: (a) PHPT and cancer can coexist, and (b) some cancers produce ectopic PTH (rare but possible).

Feature	Multiple Myeloma	Carcinoma Bone Metastases
ALP	Normal (purely lytic, no osteoblastic response) ^[8]	Usually ↑ (osteoblastic/mixed response)
Bone scan (scintigraphy)	May be NEGATIVE (bone scan detects osteoblastic activity; myeloma has none)	Positive (hot spots)
Globulin	↑ (paraprotein)	Normal
Best imaging	Skeletal survey (X-ray) or low-dose whole-body CT or PET-CT	Bone scan + CT/MRI

Why is bone scan negative in myeloma?

Bone scan (Tc-99m MDP) works by detecting osteoblastic activity — the radiotracer is adsorbed onto bone surfaces where new bone is being formed ^[15]. In myeloma, the disease is purely lytic with no compensatory osteoblastic response, so the bone scan may be falsely negative despite extensive skeletal disease. This is why skeletal survey or PET-CT is preferred for myeloma staging, not bone scintigraphy.

Since hypercalcemia presents with non-specific symptoms (confusion, dehydration, constipation, bone pain), consider other metabolic causes of the same clinical picture:

Symptom Complex	Differential Besides Hypercalcemia
Confusion + dehydration	Hypernatraemia, hyponatraemia, uraemia, hepatic encephalopathy, sepsis, drug intoxication, delirium workup (Ix: CBC, L/RFT, electrolytes including Ca/PO₄, urinalysis, blood glucose, CXR, ECG) ^[16]
Polyuria + polydipsia	Diabetes mellitus, diabetes insipidus (central or nephrogenic), primary polydipsia
Constipation	Hypothyroidism, hypokalaemia, CCB use, cord compression, bowel obstruction ^[17]
Bone pain in elderly	Osteoporotic fracture, metastatic bone disease, myeloma, Paget's disease, osteomalacia
Psychiatric symptoms + low mood	Thyroid disease, Cushing's/Addison's, parathyroid disease are important metabolic causes of mood disturbance that must be excluded ^[18]
Renal stones	Distal RTA (type 1 — hypercalciuria from acidosis causing ↑ Ca resorption from bone and ↓ tubular Ca/PO₄ reabsorption) ^[19]; hyperuricosuria (gout); cystinuria; hyperoxaluria

RFT, Ca, and Glc are included in the NICE minimum investigations for dementia workup specifically to exclude hypercalcemia, Cushing's, or Addison's disease as reversible causes of cognitive decline ^[16]^[18].

Consideration	Relevance
Tuberculosis	TB is still prevalent in HK; granulomatous TB can cause hypercalcemia via ectopic 1,25(OH)₂D production. Always consider in patients with CXR abnormalities + hypercalcemia
Nasopharyngeal carcinoma (NPC)	Endemic in Southern Chinese; can cause hypercalcemia via bone metastases
Hepatocellular carcinoma (HCC)	High prevalence due to HBV; can cause hypercalcemia (PTHrP secretion or bone mets)
Lung cancer	High smoking prevalence; Squamous cell carcinoma is the classic PTHrP-secreting tumour ^[9]^[10]
Routine health screening	Extremely common in HK; PHPT is frequently discovered as an incidental finding

A systematic step-by-step approach when you confirm hypercalcemia:

Confirm genuine hypercalcemia: Correct for albumin; if in doubt, measure ionized calcium.
Check intact PTH: The single most important discriminating test.
- PTH high/normal → PHPT, FHH, lithium, tertiary HPT
- PTH low → malignancy, granulomatous disease, vitamin D excess, drugs, thyrotoxicosis, immobilization
If PTH is high: Do 24-hour urine calcium and Ca:Cr clearance ratio to exclude FHH ^[2].
If PTH is low: Check PTHrP (for HHM), 1,25(OH)₂D (for granulomatous/lymphoma), 25(OH)D (for vitamin D toxicity), SPEP/UPEP (for myeloma), TFTs (for thyrotoxicosis), and drug history (thiazides, vitamin D/A, lithium).
Targeted imaging: Based on clinical suspicion — sestamibi scan/neck US (PHPT), CXR/CT (malignancy/sarcoidosis), skeletal survey (myeloma), bone scan (metastatic carcinoma but NOT myeloma).

Key Exam Discriminators at a Glance

Test	PHPT	FHH	Malignancy (HHM)	Myeloma	Sarcoidosis
PTH	↑	Normal/↑	↓	↓	↓
PO₄	↓	Normal	↓	Normal/↑	Normal
ALP	Normal/↑	Normal	↑	Normal	Normal/↑
24h urine Ca	↑	↓	↑	↑	↑
Ca:Cr ratio	> 0.02	< 0.01	N/A	N/A	N/A
PTHrP	Negative	Negative	Positive	Negative	Negative
1,25(OH)₂D	↑	Normal	Normal/↓	Normal	↑
Globulin	Normal	Normal	Normal	↑	Normal

High Yield Summary — Differential Diagnosis of Hypercalcemia

90% rule: PHPT + Malignancy = ~90% of all hypercalcemia. PHPT dominates outpatient, malignancy dominates inpatient.
PTH is the pivotal first test: High/normal PTH → PTH-mediated (PHPT, FHH, lithium, tertiary HPT). Low PTH → Non-PTH-mediated (malignancy, granulomatous, vitamin D, drugs, thyrotoxicosis, immobilization).
Always exclude FHH before parathyroidectomy referral — 24h urine calcium, Ca:Cr clearance ratio < 0.01 = FHH (benign, no surgery).
Phosphate is low in PHPT (PTH causes phosphaturia). If phosphate is normal, question the diagnosis.
ALP is normal in myeloma despite extensive lytic bone disease (no osteoblastic response). Bone scan may be falsely negative in myeloma.
Malignancy mechanisms: PTHrP (~80%, SCC lung), osteolytic mets (myeloma, breast), ectopic 1,25(OH)₂D (lymphoma), ectopic PTH (rare).
Granulomatous disease (sarcoidosis, TB): Ectopic 1α-hydroxylase in macrophages → ↑ 1,25(OH)₂D → responds to glucocorticoids. TB is highly relevant in Hong Kong.
Drug causes: Thiazides (↑ DCT Ca reabsorption), lithium (shifts CaSR set point), vitamin D/A excess.
Thiazides cause hypercalcemia (↑ reabsorption), loop diuretics cause hypocalcemia (calciuric). Don't confuse the two.
For delirium/dementia workup: Always check Ca as part of minimum investigations to exclude reversible metabolic causes.

Active Recall - Differential Diagnosis of Hypercalcemia

References

^[1] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf — Hypercalcemia section ^[2] Senior notes: Chemical Pathology Data interpretation.pdf ^[4] Senior notes: Block A - Confused and dehydrated: hypercalcaemia; hypocalcaemia.pdf ^[5] Senior notes: Block A - Drugs and the Kidney.pdf — Lithium section ^[6] Senior notes: Block A - Clinical Pharmacology of anti-HT and anti-HF medications.pdf — Thiazide diuretics ^[8] Senior notes: Block A - An old man with bone pain and anaemia: multiple myeloma; monoclonal gammopathy.pdf ^[9] Lecture slides: Clinical manifestation of lung cancer (1).pdf ^[10] Lecture slides: Paraneoplastic Syndrome_rev1 (1).pdf ^[14] Senior notes: Block A - Chronic Kidney Disease and its Complications.pdf ^[15] Senior notes: Ryan Ho Diagnostic Radiology.pdf — Bone scan section ^[16] Senior notes: Ryan Ho Psychiatry.pdf — Delirium workup ^[17] Senior notes: Maksim Surgery Notes.pdf — Constipation DDx ^[18] Senior notes: Ryan Ho Psychiatry.pdf — Approach to low mood / dementia Ix ^[19] Senior notes: Block A - Electrolyte and Acid-Base Disorders.pdf — Distal RTA

Diagnostic Criteria, Diagnostic Algorithm, and Investigations for Hypercalcemia

Before launching into any algorithmic workup, you must confirm the calcium is truly elevated. This is a non-negotiable first step.

"Cannot use serum calcium only, can be falsely low [or high] sometimes → 50% of serum calcium bound to albumin" ^[3].

Step 1: Correct for albumin

Corrected Ca²⁺ (mmol/L) = Total Ca + [0.02 × (40 − albumin in g/L)] ^[1]^[3]
Why? 40% of total calcium is albumin-bound. If albumin is low (e.g. nephrotic syndrome, liver failure, malnutrition), total calcium appears low — but the patient's ionized (physiologically active) calcium may be perfectly normal. Conversely, dehydration may concentrate albumin and falsely elevate total calcium.

Step 2: When correction is unreliable, measure ionized calcium directly

Correction formula does not apply when albumin < 20 g/L ^[1]
Presence of multiple myeloma or dysglobulinaemia → paraproteins bind calcium unpredictably → total albumin and serum calcium relationship falls apart at the extremes ^[2] → request ionized calcium
Big IgM molecule can cause interference in the assays ^[2]

Step 3: Rule out artefact

Prolonged tourniquet → haemoconcentration → spuriously elevated total calcium
EDTA contamination (relevant for hypocalcemia but important to know as a cause of lab error) ^[2]
Repeat the sample if in doubt

High-Yield GC Exam Point

The GC lecture learning objectives explicitly state: "Interpretation of biochemical results" ^[13]. Any exam question giving you a calcium level MUST be interpreted alongside albumin. Always calculate corrected calcium or state you would check ionized calcium. This is the single most common error students make — interpreting uncorrected calcium.

Once you confirm genuine hypercalcemia, classify severity — this determines whether you manage urgently or can investigate electively:

Severity	Corrected Ca²⁺	Clinical Significance	Associated Cause
Mild	2.55–3.0 mmol/L	Often asymptomatic/incidental; can investigate outpatient	Associated with primary hyperparathyroidism ^[1]
Moderate	3.0–3.5 mmol/L	Symptomatic; needs active treatment	Either PHPT or malignancy
Severe / Dangerous	> 3.5 mmol/L	Seizure, coma, death; hypercalcemic crisis — medical emergency	Associated with malignancy ^[1]

"Hypercalcemia most alarming → confused, dehydrated, risk of cardiac arrhythmias" ^[8]

The approach to hypercalcemia follows the endocrine investigation principle:

"Sequence of investigations: (1) History and PE → (2) Baseline blood tests → (3) Screening biochemistry → (4) Confirmatory tests → (5) Imaging → (6) Invasive tests. General principle of less invasive before more invasive. Endocrine is unique in that you need biochemistry before imaging" ^[20].

This last point is critical — in endocrine, biochemistry comes BEFORE imaging. The reason is that modern imaging is so sensitive it picks up incidental lesions (incidentalomas) that may have nothing to do with the patient's condition. You need biochemical confirmation of the hormonal axis abnormality first, then image to localize.

3.1 Step-by-Step Algorithm

4. Investigation Modalities — Detailed Breakdown

These are the "baseline blood tests" and "screening biochemistry" you order for every patient:

Investigation	What You're Looking For	Interpretation
Calcium, Phosphate, ALP ^[4]^[21]	The "calcium triad"	↑ Ca, ↓ PO₄ = PTH-mediated (PHPT); ↑ Ca, normal/↑ PO₄ = non-PTH-mediated or tertiary HPT; Normal ALP in myeloma despite lytic lesions ^[8]
Intact PTH ^[4]^[21]	The single most discriminating test	High/inappropriate → PHPT, FHH, lithium, tertiary HPT; Low/suppressed → malignancy, vitamin D, granulomatous, drugs
Albumin	For calcium correction	Needed to calculate corrected calcium
Urea, Creatinine, eGFR ^[4]	Renal function	Hypercalcemia can CAUSE renal impairment (dehydration, nephrocalcinosis) AND renal failure can CAUSE hypercalcemia (tertiary HPT). GFR must be known before prescribing bisphosphonates
25(OH) Vitamin D ^[4]	Vitamin D status	Deficiency is common even in PHPT ^[4]; excess suggests toxicity; helps guide replacement
CBC ^[21]	Anaemia, abnormal WBC	Anaemia → myeloma, malignancy; leukocytosis → haematological malignancy
LFT	Liver function, albumin	ALP component; albumin for correction
ECG	Cardiac effects of hypercalcemia	Short QT interval; check for arrhythmias; assess digoxin toxicity risk

GC Lecture High-Yield: Evaluation Panel for Asymptomatic PHPT

The GC lecture explicitly lists the evaluation panel for asymptomatic primary hyperparathyroidism ^[4]^[13]:

Calcium, Phosphate, ALP
Urea, Creatinine, eGFR
25-OH Vitamin D → deficiency is common
PTH by second or third-generation immunoassay
3-site DEXA for BMD (lumbar spine / hip / distal one-third radius)
Vertebral spine assessment (X-ray or Vertebral Fracture Assessment by DXA)

4.2 Second-Line Investigations (Guided by PTH Result)

Investigation	Rationale	Key Findings
24-hour urine calcium	To differentiate PHPT from FHH	PHPT: urinary Ca > 10 mmol/24h (typically high); FHH: urinary Ca < 2.5 mmol/24h (low)
Calcium:Creatinine Clearance Ratio (CCCR)	Gold standard to distinguish PHPT vs FHH	CCCR = (Urine Ca × Serum Cr) / (Serum Ca × Urine Cr); > 0.02 favours PHPT; < 0.01 favours FHH ^[2]; Indeterminate 0.01–0.02 → consider genetic testing
Sestamibi scan (⁹⁹ᵐTc-sestamibi)	Localize parathyroid adenoma pre-operatively	Focal uptake in abnormal parathyroid gland with delayed washout. Sensitivity ~80–90% for single adenoma; less reliable for hyperplasia or double adenoma
Neck ultrasound	Complementary localization	Hypoechoic, well-defined, oval mass posterior to thyroid. Combined with sestamibi increases sensitivity to > 95%
4D-CT parathyroid	When sestamibi + US are inconclusive	Provides perfusion characteristics of parathyroid lesions (contrast enhancement and washout patterns)
3-site DEXA ^[4]	Assess bone mineral density	Lumbar spine, hip, and distal one-third radius — the distal radius is specifically included because cortical bone is preferentially lost in hyperparathyroidism (PTH preferentially resorbs cortical > trabecular bone)
Renal ultrasound	Screen for nephrolithiasis / nephrocalcinosis	Renal stones, medullary nephrocalcinosis
Vertebral fracture assessment ^[4]	Detect occult vertebral compression fractures	By DXA or lateral spine X-ray
Drug history review	Lithium	Lithium causes hyperparathyroidism / hypercalcemia ^[5] — must be identified as a cause before considering surgery
Genetic testing	If FHH suspected or young age/family history	CaSR gene mutation for FHH; MEN1/RET mutations if MEN suspected

"If phosphate is normal in this patient, when it should be low if you are suspecting primary hyperparathyroidism → have to do 24-hour urine, exclude FHH" ^[2]. This is a critical clinical pearl — PHPT should cause low phosphate (PTH promotes renal phosphate wasting). Normal phosphate with high PTH is a red flag for FHH.

Investigation	Rationale	Key Findings
PTHrP (Parathyroid Hormone-Related Peptide)	Detect humoral hypercalcemia of malignancy	Elevated → HHM (SCC lung, RCC, breast, bladder); helps distinguish from ectopic PTH (which has elevated intact PTH, not PTHrP)
1,25(OH)₂D (Calcitriol)	Detect ectopic calcitriol production	Elevated → Lymphoma or granulomatous disease (sarcoidosis, TB); macrophage 1α-hydroxylase activity
25(OH)D (Calcidiol)	Detect exogenous vitamin D toxicity	Markedly elevated (> 375 nmol/L) → vitamin D toxicity from supplementation
SPEP and UPEP	Screen for paraprotein (M protein)	Monoclonal band in γ-region → myeloma or lymphoproliferative disease. ~50% of light chain MM is negative on SPEP → must do UPEP ^[22]
Serum Free Light Chains (FLC)	More sensitive than UPEP for monoclonal FLC	Abnormal κ:λ ratio (normal 0.26–1.65) → plasma cell dyscrasia. Involved:uninvolved FLC ratio > 100 is a myeloma-defining event ^[8]^[22]
Immunofixation	Characterize type of M protein	Follows positive SPEP/UPEP → identifies heavy chain class (IgG, IgA, IgM) and light chain type (κ or λ) ^[22]
β₂-microglobulin	Myeloma staging (ISS)	Reflects tumour burden; < 3.5 = Stage I, ≥ 5.5 = Stage III ^[22]
TFTs (TSH, FT4)	Exclude thyrotoxicosis	Suppressed TSH, ↑ FT4 → thyrotoxicosis (causes mild hypercalcemia in 15–20% via ↑ bone turnover)
ACE level	Sarcoidosis screen	Elevated in ~60% of active sarcoidosis (produced by granuloma epithelioid cells)
CXR / CT Thorax	Screen for malignancy, sarcoidosis, TB	Lung mass (SCC), bilateral hilar lymphadenopathy (sarcoidosis), upper lobe cavitation (TB)
Drug history	Thiazides, vitamin D/A supplements, lithium	Thiazides: hypercalcemia due to increased reabsorption ^[6]; vitamin A > 50,000 IU/day causes ↑ bone resorption ^[1]

Investigation	When to Order	Key Findings
Skeletal survey (X-ray)	Suspected myeloma	Punched-out lytic lesions (60%), diffuse osteopenia, pathological fractures (20%). Includes PA chest, AP/lateral C/T/L-spine, AP/lateral femur/humerus, AP/lateral skull, AP pelvis ^[22]
Whole-body low-dose CT or PET-CT	Myeloma (now preferred over skeletal survey)	Higher sensitivity than skeletal survey; PET-CT can detect active disease and monitor treatment response ^[22]
Bone scan (⁹⁹ᵐTc-MDP)	Suspected carcinoma bone metastases (NOT myeloma)	Detects osteoblastic activity — hot spots at metastatic sites. May be falsely NEGATIVE in myeloma (no osteoblastic response) ^[15]. Superscan pattern (diffuse intense uptake + absent renal uptake) seen in widespread metastases or metabolic bone disease ^[15]
Bone marrow biopsy	Suspected myeloma or haematological malignancy	Clonal BM plasma cells ≥ 10% needed for MM diagnosis ^[8]; immunophenotyping and cytogenetics for prognostic stratification
CT thorax/abdomen/pelvis	Localize occult malignancy when PTHrP is elevated	Search for primary tumour
Intraoperative PTH assay	During parathyroidectomy	PTH should drop by ≥ 50% from pre-operative baseline within 10 minutes of adenoma removal — confirms successful excision (Miami criterion)

This table is the most high-yield for data interpretation exams. Master it.

Parameter	PHPT	FHH	Malignancy (HHM/PTHrP)	Myeloma	Sarcoidosis / Granulomatous	Vitamin D Toxicity	Thyrotoxicosis
Ca²⁺	↑ (mild)	↑ (mild)	↑↑ (often severe)	↑	↑	↑	↑ (mild)
PTH	↑ or inappropriately normal	Normal or mildly ↑	↓ (suppressed)	↓	↓	↓	↓
PO₄	↓ (PTH causes phosphaturia)	Normal	↓ (PTHrP mimics PTH)	Normal or ↑	Normal	Normal or ↑	Normal
ALP	Normal or ↑	Normal	Often ↑ (bone mets)	Normal ^[8]	Normal or ↑	Normal	↑ (bone turnover)
25(OH)D	Variable (often low)	Normal	Normal	Normal	Normal	↑↑↑	Normal
1,25(OH)₂D	↑ (PTH activates 1α-hydroxylase)	Normal	Normal or ↓	Normal	↑↑	↑ or normal	Normal
PTHrP	Negative	Negative	Positive	Negative	Negative	Negative	Negative
24h urine Ca	↑	↓	↑	↑	↑	↑	↑
CCCR	> 0.02	< 0.01	N/A	N/A	N/A	N/A	N/A
Chloride	↑ (mild hyperchloraemic)	Normal	Normal	Normal	Normal	Normal	Normal
Globulin	Normal	Normal	Normal	↑↑	Normal/↑	Normal	Normal

Data Interpretation Exam Approach

When presented with a calcium data interpretation question:

Calculate corrected calcium (always — even if not asked).
Look at PTH — this splits the differential immediately.
Look at phosphate — low in PTH-mediated (except FHH); normal/high in non-PTH (except PTHrP-mediated).
Look at ALP — normal in myeloma; elevated in bone mets, Paget's, osteomalacia.
Look at globulin/albumin ratio — reversed A:G ratio → think myeloma.
If PTH is high and phosphate is normal → suspect FHH, do 24h urine ^[2].

6. Specific Diagnostic Criteria

Not all PHPT patients need surgery. The 2022 5th International Workshop guidelines for asymptomatic PHPT recommend surgery if ANY ONE of the following is present:

Criterion	Threshold
Serum calcium	> 0.25 mmol/L (1 mg/dL) above the upper limit of normal
Skeletal involvement	T-score ≤ −2.5 at any site (lumbar spine, total hip, femoral neck, distal one-third radius ^[4]); vertebral fracture on imaging
Renal involvement	eGFR < 60 mL/min; 24h urine calcium > 10 mmol/day (> 400 mg/day) AND ↑ stone risk by biochemical stone risk analysis; nephrolithiasis or nephrocalcinosis on imaging
Age	< 50 years old

If none of the above criteria are met, the patient can be monitored conservatively with annual serum calcium, creatinine, and periodic DEXA.

Hypercalcemia is one of the CRAB criteria that define myeloma as an end-organ damaging event ^[8]:

"HyperCalcaemia: Serum calcium level > 0.25 mmol/L (> 1 mg/dL) higher than the upper limit of normal or > 2.75 mmol/L (> 11 mg/dL)" ^[8]

Full IMWG 2014 criteria for MM:

Clonal bone marrow plasma cells ≥ 10% or biopsy-proven bony/extramedullary plasmacytoma, AND
One or more myeloma-defining events:
- CRAB: Calcium ↑, Renal insufficiency (Cr > 177 μmol/L or CrCl < 40), Anaemia (Hb < 10 g/dL or > 2 g/dL below LLN), Bone lesions (≥ 1 lytic lesion on imaging)
- OR biomarkers of malignancy: BM plasma cells ≥ 60%, involved:uninvolved FLC ratio > 100, ≥ 1 focal lesion (≥ 5mm) on MRI ^[8]

Clinical suspicion: Mild, chronic, asymptomatic hypercalcemia with normal or mildly elevated PTH, no kidney stones, family history of similar
Confirmed by: CCCR < 0.01 on 24-hour urine collection
Definitive: CaSR gene mutation analysis (if available)

Imaging Modality	Indication	What to Look For	Pitfalls
CXR	All patients with hypercalcemia of unknown cause	Lung mass (SCC, mets), hilar LAD (sarcoidosis, lymphoma), effusion, TB features	May miss small lesions
Neck US	Suspected PHPT — localization	Hypoechoic parathyroid adenoma posterior to thyroid	Cannot detect ectopic parathyroid tissue (mediastinal)
Sestamibi scan	Suspected PHPT — localization	Focal delayed washout in abnormal gland	Sensitivity lower for hyperplasia, double adenoma, small adenoma; false positive with thyroid nodules
CT thorax/abdomen/pelvis	Suspected malignancy	Primary tumour, lymphadenopathy, liver mets	Contrast may worsen renal function in hypercalcemic patients who are already dehydrated — rehydrate first
Skeletal survey	Suspected myeloma	Punched-out lytic lesions, osteopenia, pathological fractures ^[22]	Less sensitive than CT; being replaced by whole-body low-dose CT
Bone scan (⁹⁹ᵐTc-MDP)	Suspected carcinoma bone mets	Hot spots at metastatic sites	Falsely negative in myeloma (purely lytic, no osteoblastic activity) ^[15]
DEXA scan	PHPT — assess bone density	T-score at 3 sites including distal one-third radius ^[4]	Does not distinguish osteoporosis from other metabolic bone diseases
Renal US	PHPT or any cause with renal symptoms	Kidney stones, nephrocalcinosis, kidney size	Cannot reliably detect ureteric stones
PET-CT	Myeloma staging, occult malignancy	Metabolic activity in lytic lesions, lymph nodes	Expensive; radiation exposure

Remember: Biochemistry BEFORE Imaging in Endocrine

"Endocrine is unique in that you need biochemistry before imaging → Resolution is too high with our imaging nowadays → so we might find benign lesions, which are not related to the condition" ^[20]. Do NOT order a sestamibi scan or neck US before confirming biochemically that the patient has PTH-mediated hypercalcemia. An incidental thyroid nodule on US does not help — it confuses.

Confirm: Calculate corrected calcium (or ionized Ca if albumin < 20 / paraprotein)
Classify severity: Mild / Moderate / Severe → determines urgency
First-line bloods: Ca, PO₄, ALP, PTH, albumin, RFT, 25(OH)D, CBC, LFT, ECG
Branch on PTH:
- PTH high/normal → 24h urine Ca + CCCR → PHPT (> 0.02) vs FHH (< 0.01) → if PHPT: localization imaging (sestamibi + US) + DEXA + renal US + vertebral fracture assessment
- PTH low → PTHrP, 1,25(OH)₂D, 25(OH)D, TFTs, SPEP/UPEP/FLC, drug history, CXR → targeted imaging based on results
Treat cause: Surgery (PHPT), chemotherapy (malignancy), steroids (granulomatous), stop offending drug

High Yield Summary — Diagnosis and Investigations

Always correct calcium for albumin before interpretation. Use ionized Ca if albumin < 20 or paraproteinaemia.
PTH is the pivotal first branch-point — divides all hypercalcemia into PTH-mediated vs non-PTH-mediated.
PHPT evaluation panel: Ca, PO₄, ALP, Cr/eGFR, 25(OH)D, PTH, 3-site DEXA, vertebral fracture assessment, 24h urine Ca.
FHH exclusion: CCCR < 0.01 = FHH (benign, no surgery). If phosphate is normal with high PTH → must exclude FHH.
Myeloma clue: Normal ALP + lytic bone lesions + elevated globulin + anaemia. Bone scan may be falsely negative. Use skeletal survey or PET-CT.
Non-PTH pathway investigations: PTHrP (HHM), 1,25(OH)₂D (granulomatous/lymphoma), 25(OH)D (toxicity), SPEP/UPEP/FLC (myeloma), TFTs (thyrotoxicosis).
Biochemistry BEFORE imaging in endocrine — don't order sestamibi before confirming PTH-mediated disease.
Surgical indications in PHPT: Ca > 0.25 above ULN, T-score ≤ −2.5, eGFR < 60, nephrolithiasis/nephrocalcinosis, age < 50.

Active Recall - Diagnosis and Investigations of Hypercalcemia

References

^[1] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf — Hypercalcemia section ^[2] Senior notes: Chemical Pathology Data interpretation.pdf ^[3] Senior notes: Endocrine Interactive Tutorial.pdf ^[4] Senior notes: Block A - Confused and dehydrated: hypercalcaemia; hypocalcaemia.pdf ^[5] Senior notes: Block A - Drugs and the Kidney.pdf — Lithium section ^[6] Senior notes: Block A - Clinical Pharmacology of anti-HT and anti-HF medications.pdf — Thiazide diuretics ^[8] Senior notes: Block A - An old man with bone pain and anaemia: multiple myeloma; monoclonal gammopathy.pdf ^[13] Lecture slides: GC 039. Confused and dehydrated: hypercalcaemia; hypocalcaemia.pdf ^[15] Senior notes: Ryan Ho Diagnostic Radiology.pdf — Bone scan section ^[20] Senior notes: Block A - Introduction to Endocrine investigations.pdf ^[21] Lecture slides: GC 031. Back pain in an elderly woman: osteoporosis and related fractures.pdf — Lab investigations ^[22] Senior notes: Ryan Ho Haematology.pdf — Multiple Myeloma section

Management of Hypercalcemia

Management of hypercalcemia rests on two pillars that must run in parallel:

Rapid control of calcium levels — resuscitate the patient, break the vicious cycle, prevent cardiac and neurological catastrophe.
Early diagnosis and treatment of the underlying cause ^[4] — because no amount of IV fluid or bisphosphonate will fix the calcium long-term if you haven't dealt with the parathyroid adenoma or the cancer.

The urgency and aggressiveness of treatment is determined by severity and symptom status, not by the calcium number alone.

Three principles correspond to three severity tiers ^[4]:

Severity	Corrected Ca²⁺	Management Approach
Mild / Asymptomatic (< 3.0 mmol/L)	< 3.0	Avoid factors that aggravate hypercalcemia; maintain adequate hydration; avoid high calcium diet (> 1000 mg/day) ^[4] — investigate electively
Moderate (3.0–3.5 mmol/L)	3.0–3.5	Rapid control of calcium: fluid replacement, IV bisphosphonates, calcitonin, glucocorticoids, denosumab, dialysis, surgery, calcimimetics ^[4]
Severe / Dangerous (> 3.5 mmol/L)	> 3.5	Medical emergency — same as moderate but with greater urgency; ICU monitoring; dialysis if refractory

4. Detailed Treatment Modalities

"Volume expansion by means of normal saline — excretion of calcium is achieved by the inhibition of sodium reabsorption in the proximal tubule and loop of Henle. Calcium reabsorption follows sodium" ^[4].

This is the single most important first-line intervention. Here is why from first principles:

Hypercalcemia causes polyuria (nephrogenic DI) and vomiting → dehydration → ↓ GFR → ↓ renal calcium excretion → worsening hypercalcemia. Rehydration breaks this vicious cycle.
In the proximal tubule and thick ascending limb of Henle, calcium reabsorption is passively linked to sodium reabsorption. When you give normal saline, you expand the intravascular volume → suppress proximal tubular Na⁺ (and Ca²⁺) reabsorption → calciuresis (calcium is excreted with sodium).

Parameter	Detail
Fluid	Normal saline (0.9% NaCl) — NOT dextrose (provides free water, not NaCl)
Rate	200–500 mL/h (aim 4–6 L/day) ^[23]
Target urine output	100–150 mL/h ^[23]
Monitoring	Electrolytes and fluid balance — watch for hypokalaemia and hypomagnesaemia (dilution + calciuresis can cause K⁺/Mg²⁺ loss); cardiac monitoring for fluid overload ^[4]
Caution	Rate dependent on age and comorbid conditions (heart failure, CKD) — rapid infusion may push these patients into fluid overload ^[4]

Loop Diuretics — A Common Misconception

"Loop diuretics (furosemide) can be used only if the patients develop edema" ^[4]. "Don't answer loop diuretics as a treatment for hypercalcemia, since the effect is not that marked" ^[6].

The old teaching was to give "saline and furosemide" routinely. This is wrong. Furosemide is only indicated when the patient develops fluid overload or has pre-existing heart failure/renal insufficiency during saline rehydration ^[1]^[24]. Giving furosemide to a dehydrated patient will worsen volume depletion, reduce GFR further, and paradoxically worsen hypercalcemia. "Since these patients are already dehydrated, why dry them out even more" ^[4].

Loop diuretics should be used in patients with heart failure or renal insufficiency to prevent fluid overload ^[1]^[24] — this is the ONLY legitimate indication in the acute setting.

"Adsorb to the surface of bone hydroxyapatite and interfere with osteoclast-mediated bone resorption" ^[4] — "inhibit osteoclastic activity and reduce Ca release from bone" ^[7].

The name "bisphosphonate" tells you the mechanism: "bis" = two, "phosphonate" = phosphate-like groups. These molecules mimic pyrophosphate (which naturally coats bone surfaces) → they attach to hydroxyapatite → get internalized by osteoclasts during bone resorption → disrupt the mevalonate pathway inside the osteoclast → osteoclast apoptosis.

Parameter	Detail
Agents	Pamidronate or Zoledronic acid (Zometa) ^[4]
Dose (Pamidronate)	30–90 mg in 500 mL NS over 4–6 hours ^[23]
Dose (Zoledronic acid)	4 mg in 100 mL NS over 15 minutes (more convenient)
Onset	Serum calcium begins to fall in 1–2 days; maximum effect in 2–4 days ^[4]
Repeat dosing	Do not repeat before 7 days — must wait for full effect ^[4]
Indications	Moderate to severe hypercalcemia; patients still hypercalcemic despite adequate rehydration ^[4]
Contraindication	eGFR < 30–35 mL/min/1.73m² ^[4]^[23]. "Be careful of pseudo-low eGFR in dehydrated patients → sometimes after you rehydrate them the eGFR goes back up, which then allows you to use these bisphosphonates" ^[4]

Side effects of IV bisphosphonates ^[4]:

Flu-like symptoms (acute phase reaction — IL-6 release from monocytes)
Renal impairment (tubular toxicity, collapsing FSGS with high-dose IV) ^[5]
Osteonecrosis of the jaw (prolonged use — impaired bone healing after dental procedures)
Atypical fractures (with prolonged use for osteoporosis — related to over-suppression of bone turnover; less relevant in acute hypercalcemia treatment)

"Relatively weak agent but works rapidly within hours" ^[4]. Calcitonin is used as a bridge — it brings calcium down quickly while you wait for bisphosphonates to take effect (which take 1–2 days).

Mechanism: Increases renal calcium excretion + reduces bone resorption (directly inhibits osteoclasts) ^[4]
Dose: Salmon calcitonin 4 IU/kg SC every 12 hours ^[4]^[23]
Onset: 2–3 hours ^[23]
Key limitation: Tachyphylaxis → waning effect after 2–3 days ^[4]^[23] — the receptors downregulate. So calcitonin is a short-term measure, NOT a long-term solution.
Side effects: Nausea, flushing, hypersensitivity reaction (rare) ^[4]
Route: Subcutaneous injection. Nasal sprays are not efficacious for the treatment of hypercalcemia (nasal calcitonin is only for pain control in osteoporosis) ^[4]
Indication: Not first-line. For those contraindicated for IV bisphosphonates (e.g. severe renal impairment) or as a bridge while waiting for bisphosphonate onset ^[4]

Think of it this way: calcitonin is a "fast but weak" drug, bisphosphonates are "slow but strong." You use calcitonin to buy time.

"Reduce calcitriol production by activated mononuclear cells" ^[4] — "inhibit Vit D conversion to calcitriol" ^[23].

Glucocorticoids suppress the 1α-hydroxylase enzyme in activated macrophages within granulomas or lymphoma tissue. This makes them specifically useful when hypercalcemia is driven by excess calcitriol (1,25(OH)₂D) production.

Parameter	Detail
Agent	Prednisolone 20–40 mg/day ^[4] or IV hydrocortisone if acutely unwell ^[23]
Indications	Excessive administration or ingestion of vitamin D ^[4]; Endogenous overproduction of calcitriol (chronic granulomatous disease — sarcoidosis, TB; lymphoma) ^[4]; Haematologic malignancy ^[23]
Mechanism	Suppresses macrophage 1α-hydroxylase → ↓ calcitriol → ↓ intestinal Ca absorption + ↓ bone resorption ^[4]
Onset	Days
Limitations	NOT effective in PHPT or PTHrP-mediated malignancy (these are not calcitriol-driven); chronic use has all the standard steroid side effects

When Are Glucocorticoids Indicated for Hypercalcemia?

Only when the hypercalcemia is vitamin D–mediated:

Granulomatous disease (sarcoidosis, TB) — macrophages making excess 1,25(OH)₂D
Lymphoma — tumour cells making excess 1,25(OH)₂D
Vitamin D toxicity (exogenous)
Haematological malignancy (myeloma — also part of chemotherapy regimen)

If the cause is PHPT or solid tumour PTHrP, glucocorticoids will NOT help.

"Denosumab" → "deno" + "su" + "mab" → fully human monoclonal antibody. It targets RANKL (Receptor Activator of Nuclear Factor κB Ligand).

"RANKL causes increased formation, function and survival of osteoclasts → causes more bone resorption and increased calcium. So inhibit this, less osteoclasts → less bone resorption, calcium levels fall" ^[4].

Parameter	Detail
Agent	Denosumab 120 mg SC ^[23]
Onset	2–4 days for serum calcium to fall ^[4]
Key advantage	Not excreted through kidneys — can be given to CKD patients ^[4]. This is its major advantage over bisphosphonates
Indications ^[4]	1) Refractory hypercalcemia despite IV bisphosphonates; 2) Contraindicated for IV bisphosphonates due to severe renal impairment (eGFR < 30); 3) Hypercalcemia of malignancy with persistent hypercalcemia
Precautions	Must ensure vitamin D is replenished before starting → or else may cause hypocalcemia ^[4]; risk of rebound hypercalcemia if stopped abruptly (RANKL rebounds)

Denosumab is not first-line — it is reserved for bisphosphonate failure or renal impairment ^[4].

"Calci-mimetic" → "mimics calcium." Cinacalcet is an allosteric activator of the calcium-sensing receptor (CaSR).

"Act on CaR to switch off PTH synthesis" ^[14]. By making the CaSR more sensitive to ambient calcium, the parathyroid gland "thinks" calcium is higher than it really is → suppresses PTH secretion.

Parameter	Detail
Agent	Cinacalcet (oral)
Mechanism	Allosteric activation of CaSR → ↓ PTH secretion
Indications	Secondary/tertiary hyperparathyroidism in CKD ^[14]; Parathyroid carcinoma; Severe hypercalcemic patients in whom parathyroidectomy is indicated but clinically not a good surgical candidate ^[1]^[24]
Not used for	Mild PHPT (surgery is preferred); non-PTH-mediated hypercalcemia (pointless — PTH is already suppressed)

"Reserved for very severe forms of hypercalcemia → serum Ca > 4.5 mmol/L. Usually in patients with refractory severe hypercalcemia, complicated by renal failure → renders other forms of therapy ineffective or contraindicated" ^[4].

Parameter	Detail
Modality	Haemodialysis with low-calcium dialysate ^[7]^[23]
Mechanism	Mechanically removes plasma calcium across the dialysis membrane down its concentration gradient ^[7]
Indication	Refractory severe hypercalcemia (> 4.5 mmol/L); concurrent acute kidney injury or established CKD making other therapies ineffective
Setting	ICU — "when all else fails, consult ICU" ^[4]

5. Treatment of the Underlying Cause

Acute calcium-lowering measures are a bridge. Definitive management requires treating the cause ^[4].

5.1 Primary Hyperparathyroidism — Surgery

Parathyroidectomy is the only curative treatment for PHPT.

Category	Criteria
ALL symptomatic patients	Any patient with symptoms attributable to hypercalcemia
Asymptomatic patients — surgery if ANY ONE of:
Age	< 50 years
Serum calcium	> 0.25 mmol/L (1 mg/dL) above upper limit of normal
Renal	CrCl reduced ≥ 30% (eGFR < 60 mL/min), or 24h urinary Ca > 400 mg/day (10 mmol/day) with ↑ stone risk, or nephrolithiasis/nephrocalcinosis on imaging
Bone	Osteoporosis: DEXA T-score ≤ −2.5 at any site, or vertebral fracture
Other indications	Persistent or recurrent PHPT; Familial PHPT; Parathyroid carcinoma; Parathyroid crisis ^[1]^[24]

Contraindication	Rationale
Known recurrent laryngeal nerve (RLN) injury	Bilateral RLN injury can be life-threatening (bilateral vocal cord paralysis → airway obstruction)
Symptomatic cervical disc disease	Positioning for surgery may exacerbate
Familial Hypocalciuric Hypercalcemia (FHH)	Patient does not have PHPT; surgical intervention does not result in cure ^[1]^[24] — the problem is a systemic CaSR defect, not an adenoma

Procedure	Indication	Details
Focused (minimally invasive) parathyroidectomy	Adenoma identified on pre-operative localization (sestamibi + US)	Small incision (< 3 cm); remove single adenoma; intraoperative PTH measurement (Miami criteria): PTH drop to normal range + < 50% of max value at 10 min post-resection ^[7]. If criteria not met → suspect multigland disease → convert to bilateral neck exploration
Bilateral neck exploration (BCE)	MEN1/2A; multigland disease; uncertain imaging	Kocher incision; subtotal parathyroidectomy ("3.5 resection") — 3 glands resected, half of 4th gland left in situ marked with non-absorbable sutures ^[7]
Total parathyroidectomy with autotransplantation	Rarely needed; tertiary HPT	Autotransplant to brachioradialis (forearm) or SCM (neck) ^[7]
± Cervical thymectomy	MEN1 — to resect supernumerary glands in thymus ^[7]

Complication	Mechanism / Details
Hypocalcemia (most important to monitor)	Routinely check Ca level on post-op Day 1 ^[7]
Hungry bone syndrome	Rapid, profound hypocalcemia due to sudden drop in PTH, causing rapid deposition of Ca into demineralized bone ^[7]; more common after prolonged severe PHPT. Treatment: IV then oral calcium + vitamin D
Transient hypoparathyroidism	After focused parathyroidectomy — transient suppression of normal glands by adenoma ^[7]; usually recovers in days to weeks
Permanent hypoparathyroidism	Requiring Ca/Vit D supplement 1 year post-op ^[7]
RLN injury	Hoarseness; risk higher in re-operative surgery
Reactionary haemorrhage	Neck haematoma → airway compromise → emergency
Persistent HPT (< 6 months post-op)	Due to missed pathology → management: BCE ^[7]
Recurrent HPT (> 6 months post-op)	Due to missed pathology or parathyromatosis (disseminated parathyroid tissue from rupture during surgery) → management: MIBI scan, BCE ^[7]

Indications: Surgically unfit patients

Agent	Mechanism
Bisphosphonates	↓ Bone resorption → may stabilize BMD, modest effect on calcium
Calcimimetics (Cinacalcet)	↓ PTH secretion via CaSR activation → lowers calcium
SERM (Raloxifene)	Bone-protective (anti-resorptive); modest effect

"Persistent autonomous hypercalcaemic hyperparathyroidism after renal replacement therapy" ^[7].

Medical: Calcimimetics (cinacalcet); dietary phosphate restriction; phosphate binders; vitamin D analogues ^[14]
Surgical indications: Persistent severe hypercalcemia; impaired graft function; progressive symptoms (e.g. osteoporotic fracture) ^[7]
Procedures: Total parathyroidectomy with autotransplantation at forearm (brachioradialis) or neck (SCM); or subtotal parathyroidectomy (3.5 resection) ^[7]

Definitive treatment: Treat the underlying malignancy (chemotherapy, radiotherapy, surgery depending on tumour type and stage)
Supportive: Bisphosphonates (zoledronic acid preferred in malignancy) or denosumab for ongoing bone protection
Glucocorticoids: For lymphoma (both anti-tumour and anti-calcitriol effect) and myeloma (dexamethasone is part of chemotherapy)
Prognosis: Malignant hypercalcemia is a poor prognostic sign (median survival ~30 days if untreated); treatment of hypercalcemia improves quality of life but may not extend survival significantly

Glucocorticoids are first-line — suppress macrophage 1α-hydroxylase
Treat underlying disease (anti-TB therapy for TB, immunosuppression for sarcoidosis)

Stop the offending drug:
- Thiazides → switch to alternative antihypertensive (loop diuretic, CCB, ACEi)
- Lithium → "Treatment is to stop the drug immediately" ^[25] — discuss with psychiatry about alternatives (valproate, carbamazepine)
- Vitamin D/calcium supplements → withhold
- Vitamin A → stop supplement

"Management (SAQ!)" ^[23] — this is frequently tested. Know this cold.

Step-by-step acute management of moderate-to-severe hypercalcemia ^[23]^[4]^[7]:

Monitor: I/O chart, electrolytes, RFT, cardiac monitoring (ECG)
Withhold offending agents: Stop Ca and Vit D supplements; stop thiazide diuretics
Rehydrate: IV NS infusion 200–500 mL/h (4–6 L/day); aim urine output 100–150 mL/h
Loop diuretics: IV furosemide 20–40 mg ONLY AFTER adequate rehydration — only if volume overloaded or HF
IV Bisphosphonates (if eGFR > 30): Pamidronate 30–90 mg in 500 mL NS over 4–6h; onset 24–72h; do NOT repeat before 7 days
Calcitonin (bridge / if renal impairment): SC salmon calcitonin 4 IU/kg Q12h; onset 2–3h; risk of tachyphylaxis after 2–3 days
Glucocorticoids: Only if vitamin D–mediated cause (granulomatous disease, lymphoma, vitamin D toxicity)
Denosumab: If bisphosphonate-refractory or eGFR < 30
Calcimimetics (Cinacalcet): For secondary/tertiary HPT
Dialysis with low-Ca dialysate: Last resort for refractory severe hypercalcemia (Ca > 4.5) with renal failure
Treat underlying cause

Agent	Onset	Duration	Mechanism	Key Indication	Key Limitation
IV NS	Immediate	While infusing	↑ GFR + ↓ proximal Na/Ca reabsorption	ALL hypercalcemia	Fluid overload in HF/CKD
Furosemide	Minutes	Hours	Blocks NKCC2 → ↓ Ca reabsorption in TAL	ONLY if volume overloaded	Not a primary treatment for hypercalcemia ^[6]
Pamidronate / Zoledronic acid	1–2 days	Weeks	↓ Osteoclast-mediated bone resorption	Moderate-severe, excessive bone resorption	eGFR < 30 ^[4]; wait 7 days before repeat
Calcitonin	2–3 hours	48–72h	↑ Renal Ca excretion + ↓ osteoclast activity	Bridge; renal impairment	Tachyphylaxis after 48–72h ^[4]
Prednisolone	Days	While on treatment	↓ Macrophage 1α-hydroxylase → ↓ calcitriol	Granulomatous, lymphoma, Vit D excess	Ineffective in PHPT/PTHrP
Denosumab	2–4 days	Weeks–months	Anti-RANKL → ↓ osteoclast formation	Bisphosphonate failure; eGFR < 30	Replenish Vit D first ^[4]; rebound hypercalcemia on cessation
Cinacalcet	Hours–days	While on treatment	CaSR activation → ↓ PTH	Secondary/tertiary HPT; parathyroid CA	Ineffective in non-PTH causes
Dialysis	Immediate	During session	Mechanical Ca removal	Ca > 4.5; refractory + renal failure	Invasive; ICU setting

High Yield Summary — Management of Hypercalcemia

Acute management (SAQ): Monitor ECG and I/O → Stop Ca/Vit D supplements and thiazides → IV NS 200–500 mL/h → Furosemide ONLY if volume overloaded → IV bisphosphonate (onset 1–2 days, eGFR must be > 30) → Calcitonin as bridge (onset 2–3h, tachyphylaxis after 48–72h) → Glucocorticoids ONLY for vitamin D–mediated causes → Denosumab if bisphosphonate fails or eGFR < 30 → Dialysis as last resort (Ca > 4.5 with renal failure).

Furosemide misconception: NOT a primary treatment for hypercalcemia. Only for volume overload during rehydration.

Bisphosphonate caution: Contraindicated in eGFR < 30; beware pseudo-low eGFR from dehydration — rehydrate first, reassess eGFR, then decide.

Surgery for PHPT: Indications — all symptomatic patients; asymptomatic if age < 50, Ca > 0.25 above ULN, eGFR < 60, nephrolithiasis/nephrocalcinosis, T-score ≤ −2.5. Contraindicated in FHH (no cure from surgery), bilateral RLN injury.

Post-parathyroidectomy: Watch for hungry bone syndrome (profound hypocalcemia from calcium rushing into demineralized bone).

Tertiary HPT: Cinacalcet first-line medical; surgery if persistent severe hypercalcemia or progressive symptoms.

Treat the cause: All acute measures are bridges. Definitive management = parathyroidectomy (PHPT), chemotherapy (malignancy), steroids + anti-TB (granulomatous), stop drug (drug-induced).

Active Recall - Management of Hypercalcemia

References

^[1] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf — Hypercalcemia Treatment section ^[2] Senior notes: Chemical Pathology Data interpretation.pdf ^[4] Senior notes: Block A - Confused and dehydrated: hypercalcaemia; hypocalcaemia.pdf ^[5] Senior notes: Block A - Drugs and the Kidney.pdf — Bisphosphonates section ^[6] Senior notes: Block A - Clinical Pharmacology of anti-HT and anti-HF medications.pdf — Thiazide diuretics ^[7] Senior notes: Maksim Surgery Notes.pdf — Parathyroid section ^[8] Senior notes: Block A - An old man with bone pain and anaemia: multiple myeloma; monoclonal gammopathy.pdf ^[13] Lecture slides: GC 039. Confused and dehydrated: hypercalcaemia; hypocalcaemia.pdf ^[14] Senior notes: Block A - Chronic Kidney Disease and its Complications.pdf — CKD-MBD treatment ^[23] Senior notes: Maksim Medicine Notes.pdf — Hypercalcemia management ^[24] Senior notes: MBBS Final MB (Surgery) (Felix PY Lai).pdf — Parathyroid surgery section ^[25] Senior notes: Block A - Two cases of polyuria and polydipsia.pdf — Lithium section

Complications of Hypercalcemia

Complications of hypercalcemia can be divided into two conceptual groups: (A) complications of the hypercalcemia itself — the direct damage that elevated calcium inflicts on organ systems, and (B) complications of the treatment — iatrogenic problems arising from the therapies we use to bring calcium down. Both are important for exams.

1. Complications of Hypercalcemia Itself (Organ-System Approach)

The overarching principle is that calcium is a membrane stabilizer. At normal levels it fine-tunes excitability; at excess levels it reduces neuronal, muscular, and cardiac excitability (paradoxically — because extracellular calcium raises the threshold potential, making it harder for cells to depolarize). Simultaneously, intraluminal calcium precipitates in kidneys and soft tissues, and the hypercalciuria-dehydration vicious cycle causes progressive multi-organ damage.

Complication	Pathophysiology
Nephrolithiasis (renal stones) ^[13]^[26]	Hypercalciuria → supersaturation of calcium oxalate/phosphate in tubular fluid → stone nucleation and growth. Renal stone is the most common (MC) complication ^[7]
Nephrocalcinosis ^[13]^[26]	Diffuse calcium phosphate deposition within renal parenchyma (medullary > cortical). Unlike stones (which are in the collecting system), nephrocalcinosis is within the tissue itself → progressive tubulointerstitial damage
Nephrogenic diabetes insipidus (polyuria/polydipsia)	Calcium interferes with ADH (vasopressin) action at the collecting duct — it activates the CaSR on the thick ascending limb, inhibiting NKCC2 and disrupting the medullary concentration gradient. Also, calcium deposits damage medullary interstitium → impaired urinary concentrating ability → polyuria → dehydration
Acute kidney injury (pre-renal)	Dehydration from polyuria + vomiting → ↓ renal perfusion → ↓ GFR. "With patient's history, also have to be immediately alerted to possibility of renal failure" ^[8]
Chronic kidney disease	Long-standing nephrocalcinosis → chronic tubulointerstitial nephropathy → progressive irreversible renal scarring. Also, chronic PHPT itself is associated with CKD progression
Renal tubular dysfunction ^[26]	Calcium deposits in tubular cells → impaired tubular reabsorption/secretion → type 1 (distal) RTA-like picture, concentrating defect

"Renal complications: renal stone and nephrocalcinosis, hypertension, renal failure" ^[13]

The Vicious Cycle — From Complication Back to Cause

Renal complications of hypercalcemia feed back into the problem: nephrocalcinosis damages the kidney → ↓ GFR → ↓ renal calcium excretion → worsening hypercalcemia. Dehydration from nephrogenic DI does the same. This is why patients can present in hypercalcemic crisis — the cycle accelerates until the patient becomes acutely confused, profoundly dehydrated, and at risk of cardiac arrest.

Complication	Pathophysiology
Shortened QT interval	↑ Extracellular Ca²⁺ accelerates phase 2 repolarization (plateau phase) of the cardiac action potential → shortened ST segment → shortened QT. This is the earliest and most characteristic ECG change
Cardiac arrhythmias	"Risk of cardiac arrhythmias" ^[8]. Shortened refractory period → re-entrant circuits → ventricular tachycardia, ventricular fibrillation. Also, hypercalcemia enhances automaticity of ectopic pacemakers
Bradycardia and heart block	In severe hypercalcemia (> 4.0), calcium can slow AV conduction → first-degree AV block → complete heart block
Cardiac arrest	Extreme hypercalcemia (> 4.5–5.0) → pulseless electrical activity or asystole
Digitalis sensitivity	Hypercalcemia potentiates the effect of digoxin (both increase intracellular calcium via Na⁺/K⁺ ATPase inhibition) → ↑ risk of digoxin toxicity (arrhythmias, heart block) at therapeutic digoxin levels. Clinical pearl: check calcium in any patient on digoxin who develops toxicity
Hypertension ^[13]^[26]	Calcium → vasoconstriction of vascular smooth muscle; also, chronic volume contraction activates RAAS. "Hypertension" is listed as a renal/cardiovascular complication of PHPT ^[13]
Left ventricular hypertrophy (LVH) ^[26]	Secondary to chronic hypertension and direct effects of PTH/calcium on myocardium
Intimal/vascular calcification ^[26]	"Intimal calcification" — metastatic calcification in arterial walls when calcium-phosphate product is elevated (especially in renal failure with tertiary HPT)

Complication	Pathophysiology
Osteoporosis / osteopenia ^[13]^[26]	Chronic ↑ PTH or ↑ bone resorption → net bone loss. "Continuously ↑ PTH → cortical > trabecular bone resorption → demineralization with weakening and fracture" ^[26]. In PHPT, cortical bone (distal radius, hip) is preferentially affected; trabecular bone (spine) may be relatively preserved
Pathological fractures ^[13]^[26]	Weakened bone → fracture with minimal trauma. "2–3× risk of vertebral, distal forearm, pelvic fractures" ^[26]
Osteitis fibrosa cystica ^[26]	Classical bone lesion of severe PHPT (now uncommon with early detection). Includes: brown tumours (aggregations of osteoclasts + fibrous tissue + haemosiderin in jaw, long bones, ribs); subperiosteal bone resorption (thinning of cortex, especially radial aspect of middle phalanges); bone cysts (central medullary portions); "salt-and-pepper" skull (trabecular resorption in skull vault); tapering of distal clavicles ^[26]
Chondrocalcinosis / pseudogout (CPPD)	Calcium pyrophosphate crystals deposit in articular cartilage → joint pain, calcification of cartilage ^[13]; can cause acute inflammatory arthritis (pseudogout)

Complication	Pathophysiology
Peptic ulcer disease	Hypercalcemia stimulates gastrin secretion → ↑ gastric acid → mucosal damage. "Abdominal groans: due to ileus, PUD (gastrin secretion), pancreatitis" ^[7]
Acute pancreatitis	Calcium activates trypsinogen to trypsin within pancreatic ducts → autodigestion. Also, calcium precipitates in pancreatic ducts. Chronic hypercalcemia is a recognized cause of both acute and chronic pancreatitis
Constipation and ileus	Hypercalcemia → ↓ smooth muscle contractility → hypomotility → constipation → paralytic ileus in severe cases
Nausea, vomiting, anorexia	Central (CTZ stimulation) and peripheral (gastrin-mediated acid secretion, smooth muscle dysfunction) effects. These contribute to dehydration and worsen the vicious cycle

Complication	Pathophysiology
Confusion and cognitive impairment	"Confused and dehydrated" ^[13] — elevated extracellular calcium raises the neuronal depolarization threshold → ↓ neuronal excitability → altered consciousness. Mild hypercalcemia causes subtle cognitive dysfunction; severe causes frank delirium
Depression and mood disturbance	Central neurotransmitter effects of calcium; documented association between PHPT and depressive symptoms (often improve after parathyroidectomy)
Psychosis	Severe hypercalcemia can cause hallucinations, paranoia, disordered thinking
Lethargy, fatigue, proximal myopathy	Membrane stabilization → ↓ neuromuscular excitability → subjective weakness and objective proximal muscle weakness
Coma and death ^[1]	Extreme hypercalcemia (> 4.5–5.0) → diffuse neuronal depression → obtundation → coma. "Seizure, coma, death" associated with severe/dangerous hypercalcemia ^[1]

Complication	Pathophysiology
Band keratopathy	Calcium phosphate deposition in the interpalpebral corneal epithelium (where pH is slightly higher due to CO₂ evaporation → less soluble calcium phosphate precipitates). Visible as a horizontal white band on slit-lamp examination. Occurs in chronic hypercalcemia
Soft tissue calcification	When the calcium × phosphate product is elevated (especially > 4.4 mmol²/L² or > 55 mg²/dL²), metastatic calcification occurs in kidneys, blood vessels, lungs, heart, skin, and joints
Conjunctival calcification	Red eye from calcium deposition in conjunctiva

2. Complications of Hypercalcemia Treatment

Complication	Mechanism	Prevention
Fluid overload / pulmonary oedema	Aggressive saline infusion in patients with impaired cardiac or renal reserve → volume overload	Rate dependent on age and comorbid conditions (heart failure, CKD) ^[4]; monitor I/O, JVP, lung auscultation; use furosemide ONLY if volume overloaded
Hypokalaemia	Volume expansion + calciuresis → increased distal Na⁺ delivery → K⁺/Na⁺ exchange → potassium wasting	Monitor electrolytes; replace K⁺ as needed
Hypomagnesaemia	Similar mechanism to hypokalaemia; increased urinary magnesium losses with volume expansion	Monitor Mg²⁺; replace if low

Complication	Mechanism	Clinical Relevance
Renal impairment / nephrotoxicity	Tubular toxicity in patients with reduced renal function may occur with IV high-dose ^[5]; also risk of collapsing FSGS associated with IV high-dose ^[5]	Contraindicated when eGFR < 30–35 ^[4]; longer infusion duration (2–24h) may reduce risk ^[26]
Acute phase reaction (flu-like symptoms)	IL-6 and TNF-α release from monocytes upon first bisphosphonate exposure	Usually self-limiting; more common with first dose; paracetamol for symptom relief
Osteonecrosis of the jaw (ONJ)	Suppression of bone remodelling in the jaw (which has high turnover) → avascular necrosis after dental procedures	Exceedingly rare complication ^[27]; risk higher with prolonged IV use (oncology doses); dental assessment before starting
Atypical femoral fractures	Over-suppression of bone turnover → accumulation of microdamage → stress fracture in subtrochanteric femur	More relevant to prolonged oral bisphosphonate use for osteoporosis than acute hypercalcemia treatment ^[27]
Hypocalcaemia ^[27]	Effective suppression of osteoclast-mediated calcium release from bone → overshoot	Monitor calcium post-treatment; ensure vitamin D is replenished

Complication	Mechanism
Tachyphylaxis	Receptor downregulation after 48–72h → waning effect → calcium begins to rise again. This is the key limitation — calcitonin is a bridge, not a sustained treatment ^[4]
Nausea	Direct GI side effect
Hypersensitivity reaction	Rare; salmon calcitonin is a foreign protein

Complication	Mechanism
Hypocalcaemia	Very effective osteoclast suppression → may cause overshoot hypocalcemia, especially if vitamin D is deficient. "Have to ensure vitamin D is replenished before starting → or else, may cause hypocalcemia" ^[4]
Rebound hypercalcemia	On cessation, RANKL rebounds → sudden surge of osteoclast activity → rapid bone resorption → severe rebound hypercalcemia. Requires gradual transition to bisphosphonate if stopping
ONJ and atypical fractures	Same mechanism as bisphosphonates (suppression of bone turnover) but risk profile may differ

This is especially high-yield for surgery and endocrine exams.

Complication	Mechanism / Detail
Hypocalcaemia (most important)	Routinely check Ca level on post-op Day 1 ^[7]. Three mechanisms:
— Hungry bone syndrome	"Rapid, profound hypocalcaemia due to sudden drop in PTH, causing a rapid deposition of Ca into demineralised bone" ^[7]^[26]. The chronically PTH-stimulated skeleton suddenly starts avidly taking up calcium and phosphate when PTH drops. More severe when pre-operative PTH was very high (osteitis fibrosa cystica). Management: Ca + Vit D ^[7]
— Transient suppression of normal glands	"After focused parathyroidectomy: transient suppression of normal glands by adenoma" ^[7]. The remaining normal parathyroid glands have been chronically suppressed by the adenoma's autonomous PTH output → they take days to weeks to "wake up"
— Permanent hypoparathyroidism	"Requiring Ca / Vit D supplement 1 year post-op" ^[7]. Due to inadvertent removal or devascularization of remaining glands
Recurrent laryngeal nerve (RLN) injury ^[7]^[26]	Unilateral: hoarseness (vocal cord paralysis). Bilateral: dyspnoea, stridor upon extubation → require immediate re-intubation ± tracheostomy ^[26]. Can be transient (traction) or permanent (transection)
Reactionary haemorrhage	Neck haematoma → venous obstruction → acute laryngeal oedema → airway compromise. Management: cut subcuticular stitches and strap muscle stitches to evacuate blood → call seniors for intubation ^[26]
Persistent hyperparathyroidism (< 6 months post-op)	"Due to missed pathology → management: bilateral neck exploration (BCE)" ^[7]
Recurrent hyperparathyroidism (> 6 months post-op)	"Due to missed pathology, parathyromatosis (disseminated parathyroid tissue from rupture of parathyroid gland during operation) → management: MIBI scan, BCE" ^[7]

Hungry Bone Syndrome — The Post-Op Trap

This is a must-know complication. After years of PTH-driven bone resorption, the skeleton is demineralized and "hungry" for calcium. When the adenoma is removed and PTH drops suddenly, the skeleton rapidly takes up calcium and phosphate from the blood → profound hypocalcemia (sometimes severe enough to cause tetany, seizures, or cardiac arrhythmias). Risk factors include: high pre-operative PTH, large adenoma, radiographic bone disease, vitamin D deficiency. Prevention: ensure vitamin D is replenished pre-operatively; monitor calcium closely post-operatively; have IV calcium gluconate readily available.

Since thyroidectomy can inadvertently damage or remove parathyroid glands, iatrogenic hypoparathyroidism is a key complication:

"Risk: 1–4% permanent (esp in cancer surgery as extensive dissection is required), 10–20% transient (esp in ischaemia)" ^[26]
"Reason: often due to compromise of inferior thyroid artery" ^[26] — this artery supplies ALL four parathyroid glands
Signs: CATS GO NUMB — Convulsion, Arrhythmia, Tetany, LaryngoSpasm, NUMBNESS (perioral, distal) ^[26]

High Yield Summary — Complications of Hypercalcemia

Renal (most common organ affected):

Nephrolithiasis (MC complication), nephrocalcinosis, nephrogenic DI → polyuria/dehydration, AKI (pre-renal), CKD (chronic tubulointerstitial damage).
Vicious cycle: polyuria → dehydration → ↓ GFR → ↓ Ca excretion → worsening hypercalcemia.

Cardiac:

Short QT (earliest ECG change), arrhythmias (VT/VF), heart block, cardiac arrest (Ca > 4.5).
Digitalis sensitivity — always check calcium in digoxin toxicity.
Hypertension, LVH, vascular calcification (chronic).

Skeletal (PHPT-specific):

Osteoporosis (cortical > trabecular), pathological fractures (2–3× risk), osteitis fibrosa cystica (brown tumours, subperiosteal resorption, salt-and-pepper skull), chondrocalcinosis/pseudogout.

GI: PUD (↑ gastrin), pancreatitis, constipation/ileus, nausea/vomiting.

Neuropsychiatric: Confusion ("confused and dehydrated"), depression, psychosis, proximal myopathy, coma, seizures, death.

Treatment complications:

IV saline → fluid overload, hypokalaemia, hypomagnesaemia.
Bisphosphonates → nephrotoxicity (eGFR < 30 contraindication), ONJ, atypical fractures, acute phase reaction.
Calcitonin → tachyphylaxis after 48–72h.
Denosumab → hypocalcaemia (replenish Vit D first), rebound hypercalcaemia on cessation.
Parathyroidectomy → hungry bone syndrome (profound hypocalcemia), RLN injury, haemorrhage, persistent/recurrent HPT.

Active Recall - Complications of Hypercalcemia

References

^[1] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf — Hypercalcemia section ^[4] Senior notes: Block A - Confused and dehydrated: hypercalcaemia; hypocalcaemia.pdf ^[5] Senior notes: Block A - Drugs and the Kidney.pdf — Bisphosphonates section ^[6] Senior notes: Block A - Clinical Pharmacology of anti-HT and anti-HF medications.pdf — Thiazide diuretics ^[7] Senior notes: Maksim Surgery Notes.pdf — Parathyroid complications section ^[8] Senior notes: Block A - An old man with bone pain and anaemia: multiple myeloma; monoclonal gammopathy.pdf ^[13] Lecture slides: GC 039. Confused and dehydrated: hypercalcaemia; hypocalcaemia.pdf ^[26] Senior notes: Ryan Ho Endocrine.pdf — Hyperparathyroidism and thyroidectomy complications sections ^[27] Senior notes: Block A - Back pain in an elderly woman: osteoporosis and related fractures.pdf — ONJ and AFF