• Thyrotoxic periodic paralysis (TPP) is disproportionately common in Hong Kong and East Asia — up to 2% of Asian patients with hyperthyroidism (vs. 0.1–0.2% in non-Asians), with >95% being males aged 20–39 ^[5][6]. A susceptibility locus at 17q24.3 was discovered by HKU ^[5].
• Primary aldosteronism accounts for ~5–10% of all hypertension and is an important cause of hypertension with hypokalaemia in Hong Kong clinical practice ^[7].
• Gitelman syndrome (autosomal recessive NCC mutation) is recognised as one of the more common inherited tubulopathies worldwide and should be considered in any young patient with unexplained chronic hypokalaemia, hypomagnesaemia, and hypocalciuria ^[8].
• Diuretic use (especially thiazide for hypertension management) remains the single most common cause of hypokalaemia in clinical practice in Hong Kong ^[1][3].

Understanding hypokalaemia requires grasping normal potassium physiology from first principles.

• Total body K⁺ ≈ 3,500 mmol (in a 70 kg adult).
• ~98% is intracellular (predominantly in skeletal muscle), only ~2% is extracellular ^[1][9].
• Normal serum K⁺: 3.5–5.0 mmol/L ^[1].
• The steep intracellular-to-extracellular gradient (~140 mmol/L inside vs ~4 mmol/L outside) is maintained by the Na⁺/K⁺-ATPase pump (3 Na⁺ out, 2 K⁺ in per cycle), which is the single most important determinant of the resting membrane potential.

Why does this gradient matter? The resting membrane potential of excitable cells (cardiac myocytes, skeletal muscle, neurons) is primarily determined by the K⁺ equilibrium potential (Nernst equation: E_K = –61.5 × log([K⁺]ᵢ/[K⁺]ₑ)). When extracellular K⁺ falls (hypokalaemia), the gradient steepens → the resting membrane potential becomes more negative (hyperpolarised) → cells are initially harder to depolarise (weakness) BUT paradoxically more susceptible to arrhythmias because repolarisation is delayed and the relative refractory period is prolonged.

The first step in approaching hypokalaemia is to determine whether the total body K⁺ is truly depleted, or whether there is merely a transcellular shift ^[1][3][9].

ECG changes in hypokalaemia are a favourite exam topic ^[3][9][12]:

Mnemonic for hypokalaemia ECG: Think "No Pot, No T, but a Big U" — loss of T wave height, prominent U wave, ST depression.

Contrast with hyperkalaemia ECG: peaked T waves → widening QRS → loss of P wave → sine wave → asystole ^[3]. These are essentially the opposite changes to hypokalaemia because the mechanisms are reversed.

Chronic hypokalaemia itself causes renal damage and metabolic disturbances ^[10]:

Before investigating, always consider whether the K⁺ is genuinely low:

Pseudo-hypokalaemia is the opposite of pseudo-hyperkalaemia (which is far more common). In pseudo-hyperkalaemia, haemolysis/thrombocytosis/leukocytosis releases K⁺ into the sample. In pseudo-hypokalaemia, active WBC uptake depletes K⁺ from the sample.

If the history suggests an acute shift rather than true depletion, the total body K⁺ may be normal ^[1][9]. This distinction is critical because management differs (cautious replacement only vs. full repletion).

Clinical clues to transcellular shift:

• Recent insulin administration or carbohydrate load
• β₂-agonist use (nebulised salbutamol, IV terbutaline)
• Known thyrotoxicosis with acute paralytic episode → TPP
• Refeeding after prolonged starvation
• Acute alkalosis (e.g., hyperventilation, excessive NaHCO₃ administration)
• Hypothermia

This is the single most important first step in differentiating the cause of true K⁺ depletion ^[3][14][15].

Why does the acid-base status matter?

• In metabolic alkalosis, the kidney should be retaining H⁺ and secreting K⁺ (through upregulated H⁺/K⁺-ATPase in the wrong direction, plus aldosterone-mediated K⁺ wasting). The association of hypokalaemia + alkalosis points to specific causes.
• In metabolic acidosis, the kidney should be excreting H⁺. If K⁺ is also being lost renally despite acidosis, this suggests a specific tubular defect (RTA).
• Diarrhoea causes acidosis AND K⁺ loss directly through the gut.

Spot urine K⁺ is the key discriminator: urine K⁺ > 20 mmol/L (or urine K⁺/Cr ratio > 1.5 mmol/mmol) indicates inappropriate renal K⁺ wasting; urine K⁺ < 20 mmol/L indicates the kidney is appropriately conserving K⁺ and the loss is extra-renal ^{[1][8][9][14]}.

Alternatively, the transtubular potassium gradient (TTKG) can be used:

• TTKG > 4 in the setting of hypokalaemia confirms renal K⁺ wasting ^[8][15]
• TTKG < 3 suggests appropriate renal conservation (extra-renal loss) ^[15]

TTKG formula: TTKG = (Urine K⁺ / Plasma K⁺) ÷ (Urine Osm / Plasma Osm). It estimates the K⁺ concentration in the cortical collecting duct lumen relative to plasma, correcting for downstream water reabsorption. A high TTKG means the CCD is secreting too much K⁺ for the circumstances.

This is the most common clinical quadrant and includes the highest-yield exam differentials.

Key point: In the acidosis group, check the urine anion gap (UAG = urine Na⁺ + urine K⁺ − urine Cl⁻) ^[15]:

• Positive UAG → impaired renal NH₄⁺ excretion → RTA (the kidney is failing to generate new bicarbonate)
• Negative UAG → appropriate renal NH₄⁺ excretion → GI loss (diarrhoea) (the kidney is working correctly)

Type 4 RTA causes HYPERkalaemia, NOT hypokalaemia — it is the odd one out. It is caused by hyporeninemic hypoaldosteronism, most commonly from diabetic nephropathy (destruction of the JG apparatus by vascular hyalinosis) ^[18], or from drugs (ACEI, ARB, spironolactone, trimethoprim, heparin, NSAIDs). Students often confuse RTA types — only types I and II cause hypokalaemia.

Why is ECG done first? Because the most immediately life-threatening consequence of hypokalaemia is cardiac arrhythmia. The ECG dictates the urgency of treatment — a patient with K⁺ of 2.8 and a normal ECG can receive oral replacement; the same patient with U waves merging into T waves and frequent PVCs needs IV replacement with cardiac monitoring.

Urine tests are the most important aetiological investigations in hypokalaemia. They answer the fundamental question: is the kidney appropriately conserving K⁺, or is it inappropriately wasting it?

Why is the TTKG useful? A spot urine K⁺ can be misleading if the urine is very dilute (low K⁺ concentration despite high K⁺ excretion rate) or very concentrated (high K⁺ concentration despite low excretion rate). The TTKG corrects for this by normalising urine K⁺ to the osmolality ratio, essentially estimating what the K⁺ concentration would be at the end of the cortical collecting duct before the medullary collecting duct concentrates the urine.

TTKG formula explained from first principles: The urine K⁺ that you measure has been "concentrated" by water reabsorption in the medullary collecting duct. To undo this concentration, you divide by (Urine Osm / Plasma Osm), which tells you how much water was removed. Then you compare the "corrected" CCD K⁺ to the plasma K⁺ to get the gradient. A TTKG > 4 means the CCD is actively secreting K⁺ at a rate that is inappropriately high for the serum K⁺ level ^[15].

These are ordered after the basic workup has narrowed the differential — they target specific aetiologies.

This is a high-yield sequence taught across multiple lectures ^[17][19][20]:

Key interpretation points for confirmatory and subtype tests:

These are used when RTA or inherited tubulopathy is suspected ^[8][15].

Urgent treatment is required when ^[3]:

• Serum K⁺ < 2.5 mmol/L
• ECG changes are present (U waves merging with T waves, ST depression, QT prolongation, ventricular ectopics)
• Symptomatic (significant weakness, paralysis, respiratory compromise, arrhythmia)
• Patient is on digoxin (hypokalaemia potentiates digoxin toxicity — even "mild" hypokalaemia becomes dangerous)
• Patient has active acute MI, heart failure, or hepatic encephalopathy (K⁺ must be kept > 4.0 in these settings)

Intravenous KCl is the treatment of choice for severe or symptomatic hypokalaemia ^[1][9][21].

How much K⁺ should you give? As a rough guide, a drop of 1 mmol/L in serum K⁺ corresponds to a total body deficit of approximately 200–400 mmol. However, this relationship is non-linear — at very low serum levels, small changes represent much larger total body deficits. In practice, replace empirically and recheck frequently rather than trying to calculate exact deficits.

Practical prescribing example ^[21]:

• Moderate hypoK (K⁺ 2.5–3.0): 20 mmol KCl in 1L NS over 2h (= 10 mmol/h), repeat as needed
• Severe hypoK (K⁺ < 2.5): 40 mmol KCl in 1L NS over 2h (= 20 mmol/h) with continuous cardiac monitoring; can use central line for faster rates if needed
• Recheck K⁺ after every 40–60 mmol replaced

TPP requires a fundamentally different approach to K⁺ replacement because total body K⁺ is normal — it is merely redistributed intracellularly ^[5][6].

In DKA, serum K⁺ may appear normal or even elevated on presentation due to acidosis + insulin deficiency shifting K⁺ out of cells, despite massive total body K⁺ depletion from osmotic diuresis ^[13][22].

Typical oral dosing: 40–100 mmol/day in divided doses (e.g., Slow-K 2–4 tabs TDS) for true depletion. Mild cases may need only 40 mmol/day.

Why KCl and not other potassium salts? Potassium citrate or potassium bicarbonate alkalinise the blood further — problematic when most causes of hypokalaemia (diuretics, vomiting, aldosteronism) already cause metabolic alkalosis. KCl provides the Cl⁻ needed to correct the alkalosis. Exception: in RTA (where there IS metabolic acidosis), potassium citrate or potassium bicarbonate is preferred because you WANT to provide alkali alongside K⁺.

Dietary counselling is an adjunct, not a replacement for pharmacological therapy in significant hypokalaemia:

Practical note: A single banana provides only ~12 mmol of K⁺. To correct a moderate deficit of 200+ mmol, you would need to eat ~17 bananas. This illustrates why dietary advice alone is insufficient for significant hypokalaemia — you need pharmacological supplementation. But diet is useful for prevention and maintenance.

This is the most common cause in clinical practice ^[1][3].

Why does NS fix the alkalosis? In vomiting, the kidney cannot excrete excess HCO₃⁻ because it is volume-depleted and Cl⁻-depleted (Cl⁻ is needed for HCO₃⁻ excretion via the Cl⁻/HCO₃⁻ exchanger pendrin in type B intercalated cells). Giving NS restores volume and Cl⁻ → the kidney can now excrete the excess HCO₃⁻ → alkalosis corrects → K⁺ wasting diminishes.

Treatment depends on the subtype — this is why the diagnostic workup to differentiate adenoma from hyperplasia is so important ^[17][24]:

K⁺-sparing diuretics for primary aldosteronism ^[17][24]:

Correct hypoK + control BP + manage diabetes pre-operatively, then definitive surgical treatment ^[24]:

Management and progress from the teaching clinic case ^[8]:

Treatment is primarily supportive ^[8]:

Why potassium citrate and not KCl for RTA? In RTA, the problem is metabolic ACIDOSIS, not alkalosis. Citrate is metabolised to bicarbonate in the liver, so potassium citrate provides both K⁺ and alkali therapy in one preparation. KCl would correct the K⁺ but not the acidosis.

Cardiac arrhythmia is the most dangerous acute complication of hypokalaemia, particularly when K⁺ < 2.0 mmol/L ^[3][25].

Pathophysiological mechanism — from first principles:

The cardiac action potential depends on sequential opening and closing of ion channels. During phase 3 (repolarisation), K⁺ flows OUT of the myocyte through delayed rectifier K⁺ channels (IKr, IKs), restoring the resting membrane potential. When extracellular K⁺ is low:

1. The Nernst equilibrium potential for K⁺ (E_K) becomes more negative → the resting membrane potential hyperpolarises (more negative, e.g., from –90 mV to –95 mV)
2. Paradoxically, the IKr (hERG) channels conduct LESS current at low extracellular K⁺ — this is because low extracellular K⁺ destabilises the open state of the hERG channel (a biophysical property unique to this channel). Less IKr → slower repolarisation → longer action potential duration → prolonged QT interval
3. Prolonged repolarisation creates a vulnerable window where early afterdepolarisations (EADs) can occur — spontaneous depolarisations during the relative refractory period
4. EADs can trigger polymorphic ventricular tachycardia (Torsades de Pointes, TdP) → which can degenerate into ventricular fibrillation → cardiac arrest

The rate of fall of K⁺ matters as much as the absolute level — an acute drop from 4.0 to 2.5 is more dangerous than a chronic stable K⁺ of 2.8, because the heart has had no time to adapt its channel expression in the acute scenario ^[3].

ECG changes in hypokalaemia represent a continuum of worsening repolarisation disturbance ^[3]:

Hypokalaemia potentiates digoxin toxicity — this is a critically important drug-electrolyte interaction ^[1][3].

Mechanism from first principles: Digoxin works by inhibiting the Na⁺/K⁺-ATPase on the cardiac myocyte membrane. K⁺ and digoxin compete for the same binding site on the extracellular face of the α-subunit of Na⁺/K⁺-ATPase. When extracellular K⁺ is low, fewer K⁺ ions occupy this binding site → more binding sites are available for digoxin → enhanced pump inhibition → more intracellular Na⁺ → Na⁺/Ca²⁺ exchanger shifts to import more Ca²⁺ → intracellular Ca²⁺ overload → delayed afterdepolarisations (DADs) → triggered arrhythmias (e.g., bidirectional VT, accelerated junctional rhythm, PAT with block).

Clinical implication: A patient on digoxin with "therapeutic" serum digoxin levels can still develop digoxin toxicity if K⁺ drops. Never prescribe K⁺-wasting diuretics to a patient on digoxin without close K⁺ monitoring and concurrent K⁺ supplementation or K⁺-sparing diuretics. In such patients, the target serum K⁺ should be maintained > 4.0 mmol/L.

Muscle weakness and paralysis (proximal muscle myopathy) is one of the most characteristic symptoms of hypokalaemia ^[3].

Pathophysiology:

• Hypokalaemia → hyperpolarised resting membrane potential (more negative) → the cell requires a larger depolarising stimulus to reach threshold → the muscle fibre is effectively less excitable
• Proximal muscles (quadriceps, hip flexors, shoulder girdle) are affected first because they have the highest metabolic demand and are most sensitive to changes in membrane potential
• Pattern: proximal > distal, lower limbs > upper limbs. Respiratory and bulbar muscles are usually spared except in severe cases ^[5][6]
• In extreme hypokalaemia (K⁺ < 2.0), the membrane becomes so hyperpolarised that it is essentially inexcitable → flaccid paralysis with hypotonia and areflexia

Hypokalemia complications include generalised weakness especially of the proximal musculature, respiratory failure, and fatal arrhythmia ^[25].

In very severe hypokalaemia (K⁺ < 2.0), weakness can extend to the diaphragm and intercostal muscles → respiratory failure requiring mechanical ventilation. This is a medical emergency.

This is a complication often forgotten because clinicians focus on the cardiac effects. In the teaching case discussion, respiratory failure is explicitly listed as a hypokalemia complication alongside fatal arrhythmia ^[25].

Rhabdomyolysis is a serious complication of severe hypokalaemia ^[3][10].

Pathophysiology — two complementary mechanisms:

1. Impaired blood flow to exercising muscles: K⁺ normally mediates local arteriolar vasodilation during exercise (K⁺ released from active muscle cells acts on vascular smooth muscle inward-rectifying K⁺ channels, causing hyperpolarisation → relaxation → dilation). When K⁺ is depleted, this vasodilatory response is impaired → relative ischaemia during exercise → muscle necrosis
2. Direct sarcolemmal damage: Severe K⁺ depletion alters the resting membrane potential so profoundly that ion channel dysfunction occurs in the sarcolemma → abnormal Ca²⁺ handling → sustained contraction → cell death

Clinical features: Muscle pain, tenderness, dark-coloured urine (myoglobinuria), markedly elevated CK (> 5× ULN)

Complication cascade: Rhabdomyolysis → myoglobin release → myoglobin precipitates in renal tubules (especially in acidic urine) → acute kidney injury (AKI) → which then impairs K⁺ excretion → potentially paradoxical hyperkalaemia from the combination of ongoing cell lysis and reduced renal clearance.

Polyuria is a recognised complication of hypokalaemia ^[3].

Pathophysiology — from first principles:

• K⁺ is required for the Na⁺/K⁺/2Cl⁻ cotransporter (NKCC2) in the thick ascending limb of the Loop of Henle to function. When K⁺ is depleted, NKCC2 cannot operate efficiently → less NaCl is reabsorbed into the medullary interstitium → loss of the corticomedullary osmotic gradient → the collecting duct cannot concentrate urine even if ADH is present ^[1][10]
• Additionally, chronic hypokalaemia downregulates aquaporin-2 (AQP2) expression in the collecting duct principal cells → water reabsorption is further impaired
• The result is a nephrogenic diabetes insipidus-like picture: polyuria (dilute urine despite elevated plasma osmolality) + secondary polydipsia

This effect is clinically important: patients with chronic hypokalaemia (e.g., Bartter syndrome, chronic diuretic use) often present with polyuria and nocturia that can be mistaken for primary polydipsia or central DI if the K⁺ is not checked.

Chronic hypokalaemia both generates and maintains metabolic alkalosis through several renal mechanisms ^[10][25]:

For hypokalaemia, we always check serum bicarbonate — bicarbonate would be expected to be high because hypokalaemia is usually associated with alkalosis. The sodium/hydrogen/potassium exchangers in the kidney mean that as a consequence of trying to reabsorb potassium, your body dumps out hydrogen, resulting in urine acidosis and systemic alkalosis ^[25].

This alkalosis is "maintained" by hypokalaemia: as long as K⁺ remains low, the kidney continues to generate new HCO₃⁻ and cannot excrete the excess. Correcting K⁺ is therefore essential to correcting the alkalosis.

This is a clinically important complication in cirrhotic patients:

• Hypokalaemia → intracellular acidosis in PCT → upregulated glutaminase → increased NH₃ production
• NH₃ is normally converted to urea by the liver → but in cirrhosis, the liver cannot metabolise NH₃ efficiently → NH₃ accumulates systemically → crosses the blood-brain barrier → astrocyte swelling → hepatic encephalopathy

Clinical pearl: In any cirrhotic patient with worsening encephalopathy, ALWAYS check K⁺ and correct it aggressively. Common precipitants of hepatic encephalopathy include diuretics (causing hypoK + triggering ↑ renal ammonia), GI bleeding (protein load), constipation (↓ faecal ammonia excretion), and infection. Hypokalaemia is a frequently overlooked contributor.

Chronic, severe, untreated hypokalaemia causes structural kidney damage ^[10]:

• Vacuolar nephropathy: Tubular epithelial cells develop cytoplasmic vacuolisation (especially in the PCT and medullary collecting duct)
• Chronic interstitial nephritis and fibrosis: Prolonged K⁺ depletion leads to tubular cell apoptosis, interstitial inflammation, and progressive fibrosis
• Can eventually progress to chronic kidney disease (CKD), particularly in patients with untreated Bartter syndrome

Sustained hypokalaemia and hyperreninaemia can cause progressive tubulointerstitial nephritis, resulting in end-stage kidney disease ^[8].

• Specific to certain causes of chronic hypokalaemia, particularly Bartter syndrome (hypercalciuria → calcium salt deposition in the renal medulla) and distal RTA type I (alkaline urine + hypercalciuria → Ca²⁺ phosphate precipitation)
• Surveillance renal ultrasound should be employed to monitor for the development of nephrocalcinosis — a common complication that further impairs urinary concentrating ability ^[8]

Occurs in 40–59% of TPP cases when the transcellular shift reverses and K⁺ that was driven intracellularly floods back into the extracellular compartment ^[5]. Also a risk in DKA management when acidosis is corrected.

Prevention: Cautious replacement (max 10–20 mmol/h in TPP), frequent K⁺ monitoring, cardiac monitoring.

Slow-release KCl tablets, if crushed or if lodged in the oesophagus, can cause focal mucosal ulceration due to high local K⁺ concentration. Instruct patients to swallow whole with plenty of water, in an upright position.