• Prevalence: Rare — approximately 1 in 100,000 ^[2]
• Autosomal dominant mode of inheritance ^[1][4]
• Male predominance — despite AD inheritance, non-penetrance is common especially in females (up to 50% of carrier females never have attacks, likely due to lower androgen levels and smaller muscle mass) ^[2]
• Age of onset: 10–20 years old — attacks often begin around puberty and may decrease in frequency after age 35–40, but patients can develop a fixed progressive proximal myopathy in later decades ^[2][3]

• Highest in Asian population — up to 2% of Asian patients with hyperthyroidism vs 0.1–0.2% in non-Asians ^[5][6]
• Predominantly in male patients — risk 25% (M) vs 0.8% (F), > 95% male ^[5][6]
• Age of onset: > 20 years old (20–39 years old) ^[2][5]
• Mainly in Orientals, rare in Caucasians ^[6]
• Occurs during hyperthyroidism, not when euthyroid ^[6]
• Underlying cause: majority Graves' disease but can be due to any cause (including thyroxine abuse) ^[5]
• A susceptibility locus at 17q24.3 was discovered by HKU (Cheung et al., 2012) — this is a Kir2.6 inward-rectifying potassium channel gene polymorphism ^[5]

• Causes include diuretic abuse, diuresis after alcohol binge, renal tubular acidosis, Gitelman syndrome, Bartter syndrome, hyperaldosteronism, GI losses ^[3]
• Sporadic, M > F, any age ^[3]

1. Resting membrane potential of skeletal muscle ≈ −90 mV, maintained primarily by the K⁺ equilibrium potential (Nernst equation: the ratio of intracellular to extracellular K⁺)
2. Motor neuron releases acetylcholine → nicotinic receptor activation → Na⁺ influx → depolarisation
3. When threshold is reached (≈ −55 mV), voltage-gated Na⁺ channels (Nav1.4) open → rapid depolarisation (action potential)
4. Voltage-gated Ca²⁺ channels (Cav1.1 / dihydropyridine receptor, DHPR) in the T-tubule sense the depolarisation and mechanically couple to the ryanodine receptor (RyR1) on the sarcoplasmic reticulum → Ca²⁺ release → muscle contraction
5. K⁺ channels (including Kir inward rectifiers) repolarise the membrane
6. Na⁺/K⁺-ATPase restores ionic gradients (3 Na⁺ out, 2 K⁺ in per cycle)

The key pathological endpoint is muscle membrane inexcitability:

• In hypokalemia, extracellular [K⁺] drops → the K⁺ equilibrium potential becomes more negative (more hyperpolarised) by the Nernst equation
• In normal muscle, the inward-rectifier K⁺ channels (Kir) help "clamp" the membrane potential close to E_K, so mild hypokalemia causes only mild hyperpolarisation
• In HypoPP with mutant channels, an anomalous leak current (gating pore current) develops during hypokalemia → this paradoxically causes sustained depolarisation of the membrane to around −60 mV
• At −60 mV, Nav1.4 channels enter a state of sustained inactivation — they cannot be recruited for action potential generation
• Result: the muscle fibre is electrically silenced → flaccid weakness/paralysis

Think of it like this: the muscle membrane gets "stuck" at a voltage where it can neither rest properly nor fire. It's in a "dead zone" of inexcitability.

• Na⁺/K⁺-ATPase pumps 3 Na⁺ out and 2 K⁺ in — this is the main driver of intracellular K⁺ shift
• Thyroid hormones directly increase genetic transcription of genes coding for the Na-K ATPase pump as well as increase the pump's intrinsic activity ^[6]
• Thyroid hormones also cause beta-2 adrenergic stimulation and a rise in sensitivity to circulating catecholamines, resulting in an increase in Na-K pump activity ^[6]
• Increased androgen levels, characteristically seen in males, have been associated with increased Na-K pump activity ^[6]
• Hyperinsulinemia seen in attacks of TPP may also contribute to a depletion in extracellular potassium — especially after a high carb load ^[6]

This is why in TPP, the total body potassium is normal — K⁺ is simply shifted from extracellular to intracellular compartment. This has critical management implications (rebound hyperkalemia with aggressive replacement).

• Autosomal dominant pattern ^[2]
• All mutations share a common final pathway: they create a "gating pore" — a leak current through the voltage-sensing domain of the channel that allows cations to flow aberrantly when the channel should be in a resting state
• This gating pore current is unmasked by hypokalemia (which normally would hyperpolarise and stabilise the membrane)

Pathophysiology ^[5][6]:

1. Thyrotoxicosis → direct increase in genetic transcription of Na-K ATPase pump + increase in pump's intrinsic activity
2. β₂-adrenergic hypersensitivity → thyroid hormones cause beta-2 adrenergic stimulation and a rise in sensitivity to circulating catecholamines → increase in Na-K pump activity
3. Hyperinsulinemia (especially post-carbohydrate) → insulin stimulates Na⁺/K⁺-ATPase and GLUT4 → K⁺ shifts intracellularly
4. Androgens → increased androgen levels have been associated with increased Na-K pump activity (explains male predominance)

Precipitating factors ^[5][6]:

In Hong Kong practice, important secondary causes include:

• Gitelman syndrome — relatively common in Chinese population, autosomal recessive, caused by mutations in SLC12A3 (thiazide-sensitive NCC) in DCT ^[8]
• Diuretic abuse (common in elderly HK patients on multiple medications) ^[3]
• Diuresis after alcohol binge ^[3]
• Renal tubular acidosis (type 1 distal RTA)
• Licorice ingestion — inhibits 11β-hydroxysteroid dehydrogenase → cortisol excess → mineralocorticoid receptor activation → K⁺ wasting ^[9]

This is fundamentally about the Nernst equation and membrane excitability:

Step 1: Normal state

• Resting membrane potential ≈ −90 mV
• Extracellular [K⁺] ≈ 3.5–5.0 mmol/L
• Intracellular [K⁺] ≈ 140 mmol/L
• The steep gradient (140:4 ≈ 35:1) determines E_K ≈ −90 mV

Step 2: In hypokalemia

• Extracellular [K⁺] drops to e.g. 2.0 mmol/L
• By Nernst: E_K becomes more negative (e.g. −105 mV)
• In normal muscle: the membrane simply hyperpolarises slightly → harder to reach threshold → mild weakness at most
• In HypoPP (mutant channels): paradoxical sustained depolarisation to ≈ −60 mV due to anomalous gating pore currents

Step 3: Sustained depolarisation → Inexcitability

• At −60 mV, all Nav1.4 channels enter slow inactivation
• Slow inactivation = the channel is "locked shut" and cannot be activated by further depolarisation
• No action potentials can be generated → flaccid paralysis

Step 4: Recovery

• When serum K⁺ is restored, the membrane repolarises
• Nav1.4 channels recover from inactivation
• Action potentials can fire again → weakness resolves

• Proximal muscles have higher density of Na⁺/K⁺-ATPase and are more metabolically active
• Lower limbs have greater muscle bulk → larger absolute K⁺ shift
• The diaphragm and bulbar muscles have a different fibre type composition (type I slow-twitch predominant) with different channel expression → seldom respiratory/bulbar muscle involvement ^[5][6]

• Cardiac myocytes also depend on K⁺ gradients for repolarisation
• Hypokalemia → prolonged repolarisation → prolonged QT interval ^[10]
• Creates substrate for early afterdepolarisations (EADs) → risk of Torsades de Pointes
• ECG changes: large U wave, loss/flattening of T wave, prolonged QT interval, ST depression, PR prolongation ^[10][11]
• Cardiac arrhythmia risk particularly when [K⁺] < 2.0 ^[10]

Total body potassium is actually normal or high in TPP — the K⁺ has simply shifted intracellularly ^[5][7]. When the attack resolves (spontaneously or with treatment), all that intracellular K⁺ shifts back out. If you have given IV K⁺ supplementation aggressively, you now have the original total body K⁺ PLUS the supplemented K⁺ → rebound hyperkalemia (40–59%) ^[5].

This is fundamentally different from true hypokalemia (e.g., from GI losses, diuretics) where total body K⁺ is genuinely depleted.

• Mild: Subjective weakness, able to ambulate
• Moderate: Cannot ambulate, proximal weakness 2–3/5
• Severe: Near-complete paralysis (0–1/5), cardiac arrhythmia risk, K⁺ often < 2.0

^[1][2][3][5]

• Frequency: weekly to monthly ^[5]
• Duration: minutes to days (TPP) ^[5]; 6–24 hours (primary familial HypoPP) ^[1][4]
• Course: In primary HypoPP, attacks tend to become less frequent after age 35–40 but permanent myopathy may develop
• TPP: attacks cease entirely when euthyroid state is achieved ^[6]

Both present with hypokalemic metabolic alkalosis and are relevant in the Hong Kong paediatric/young adult population ^[8]:

Why does this matter? Because Gitelman syndrome is relatively common in the Chinese population and can present with episodic weakness mimicking periodic paralysis ^[8]. The distinguishing feature is that K⁺ remains persistently low (not episodically low), and there is renal K⁺ wasting with elevated urine K⁺ and TTKG.

• Hypokalemic hypertension is the hallmark ^[15]
• Primary hyperaldosteronism → elevated aldosterone, suppressed renin
• The hypertension distinguishes it from periodic paralysis and Gitelman/Bartter
• Important in the workup of youth hypertension — if a young patient has hypertension + spontaneous hypokalemia → consider Conn syndrome, bilateral adrenal hyperplasia, pheochromocytoma, renal artery stenosis ^[15]

• Inhibits 11β-hydroxysteroid dehydrogenase → cortisol is not converted to cortisone → cortisol binds mineralocorticoid receptor → Na⁺ retention, K⁺ wasting → hypokalemia ^[9]
• Presents with hypokalemic metabolic alkalosis ± hypertension
• Important in HK because licorice is found in traditional Chinese herbal teas — history of herbal tea consumption should be specifically elicited ^[8]

The K⁺ loss from vomiting is NOT from direct GI loss through the mouth ^[9]. The mechanism is:

1. Vomiting → loss of HCl → metabolic alkalosis
2. Alkalosis → increased filtered HCO₃⁻ → increased delivery of Na⁺ + HCO₃⁻ to the distal tubule
3. Volume depletion → secondary hyperaldosteronism
4. Both mechanisms → increased renal K⁺ secretion

This is why vomiting is associated with ↑TTKG, ↑urine K⁺ — it is a renal loss of K⁺ ^[9][11].

Diagnosis requires all of the following:

Step 1: Confirm hypokalemia in the context of weakness

• Draw urgent serum electrolytes (K⁺, Na⁺, Cl⁻, HCO₃⁻, Mg²⁺, Ca²⁺, PO₄³⁻) + RFT + glucose
• Perform 12-lead ECG immediately (to assess for arrhythmia risk and hypoK changes)
• If K⁺ is normal → the weakness is NOT from hypokalemia → pursue neurological differential (GBS, MG, etc.)

Step 2: Determine if this is episodic (periodic paralysis) vs persistent hypokalemia

• Attacks of flaccid paralysis lasting 6–24 hours with marked hypoK, spontaneously returned to normoK afterwards ^[1][4] → this pattern = periodic paralysis
• If K⁺ is persistently low → this is chronic hypokalemia with weakness, not periodic paralysis → investigate cause (diuretics, RTA, Gitelman, hyperaldosteronism, etc.)

Step 3: Check thyroid function — THE critical branch point in Hong Kong

• Bloods: electrolytes (K↓↓), CPK↑, TFT ^[7]
• If TSH suppressed + fT4/fT3 elevated → TPP ^[5][7]
• If euthyroid → primary familial HypoPP or other secondary cause

Step 4: If euthyroid, use the hypokalemia diagnostic algorithm

This follows the classic approach taught in HKUMed electrolyte lectures ^[10][11][14]:

Step 4a: Assess plasma HCO₃⁻ ^[10][11][14]

Why check HCO₃⁻ first? Because it immediately splits the differential into two very different groups. Most causes of hypokalemia produce alkalosis (the H⁺/K⁺ exchange mechanism + volume contraction). Hypokalemic acidosis narrows d/dx to RTA and diarrhoea ^[11] — this is a huge diagnostic shortcut.

Step 4b: Check urinary K⁺ excretion (paired spot urine before K⁺ replacement) ^[11][14]

The TTKG (Trans-Tubular Potassium Gradient) is calculated as: TTKG = (Urine K⁺ / Plasma K⁺) × (Plasma Osm / Urine Osm) It estimates the K⁺ concentration in the cortical collecting duct, reflecting aldosterone activity. A TTKG > 7 in the setting of hypokalemia means the kidney is inappropriately wasting K⁺ ^[11].

Alternative: spot urine K/Cr > 2.5 also indicates renal K⁺ wasting ^[11].

Step 4c: Check blood pressure (if renal K⁺ wasting confirmed) ^[11][15]

Step 4d: If metabolic acidosis — use urine anion gap ^[8]

Why does the UAG work? NH₄⁺ is excreted with Cl⁻ as the counter-ion. If the kidneys are properly compensating for acidosis by excreting NH₄⁺, then urine Cl⁻ will be high (carried with NH₄⁺), making the UAG negative. If the distal tubule is dysfunctional (RTA), it cannot secrete H⁺ or NH₄⁺, so urine Cl⁻ is relatively low → UAG positive.

Critical timing point: The paired spot urine K⁺ must be collected BEFORE potassium replacement is started ^[14]. Once you give IV KCl, the urine K⁺ rises and you lose the ability to distinguish renal from extrarenal loss. This is a common exam trap.

The ECG changes of hypokalemia follow a predictable progression as K⁺ drops ^[10][11]:

ECG: hypoK → PR prolongation, ST depression, U wave ^[7]

The paired spot urine K⁺ and HCO₃⁻ must be assessed BEFORE potassium replacement ^[14]. Once exogenous K⁺ is given, urine K⁺ rises (as the kidney excretes the load) and the diagnostic utility is lost. In practice, this means:

• In the emergency department, draw blood AND urine samples simultaneously before starting any IV KCl
• Label them clearly as "pre-replacement samples"

Use this clinical decision rule:

1. Young male + Asian + episodic weakness + normoK between attacks → strongly suspect PP
2. Thyrotoxic symptoms present → TPP
3. No thyrotoxic symptoms + FHx + onset teens → Primary familial HypoPP
4. Persistent hypoK + any of: hypertension, medication use, GI symptoms, acid-base disturbance → Secondary cause

Propranolol: to blunt Na-K ATPase ^[7]

Why propranolol specifically and not a β₁-selective blocker like metoprolol? Because the target is β₂-adrenergic-mediated Na⁺/K⁺-ATPase activity in skeletal muscle. You need a non-selective β-blocker to hit β₂ receptors. A β₁-selective agent would help with heart rate but not address the muscle channelopathy mechanism.

Severe hypoMg < 0.3 mmol/L inhibits action of PTH and Na⁺/K⁺-ATPase → hypoK + hypoCa refractory to K and Ca replacement alone ^[4][19]

• If Mg²⁺ is low → replace with IV MgSO₄ (e.g., 2–4 g over 4–8 hours) before or concurrently with K⁺ replacement
• Without Mg²⁺ correction, the K⁺ simply will not stay up — the Na⁺/K⁺-ATPase cannot function properly without Mg²⁺ as a cofactor

Attacks can be prevented by ^[6][13]:

Definitive treatment: treat the underlying thyrotoxicosis ^[5][7]

Occurs during hyperthyroidism, not when euthyroid ^[6][13]. Once the patient achieves and maintains a euthyroid state, TPP attacks cease entirely. This is the definitive cure. The key message: K⁺ management is a bridge; thyroid treatment is the solution.

Why acetazolamide? This seems counterintuitive — a diuretic for a condition with K⁺ shifts. The mechanism is not fully understood, but the prevailing theory is that acetazolamide:

1. Creates systemic metabolic acidosis by inhibiting renal HCO₃⁻ reabsorption
2. Acidosis shifts K⁺ out of cells (H⁺/K⁺ exchange) → maintaining higher extracellular K⁺
3. May also directly affect muscle membrane potential through changes in intracellular pH
4. Causes carbonic anhydrase inhibition in muscle → alters local CO₂/HCO₃⁻ balance → stabilises resting potential

Acetazolamide is NOT effective in TPP because the driver is thyrotoxicosis, not an intrinsic channel mutation. It only works in primary familial HypoPP.

If K⁺ rises > 5.5 mmol/L after attack resolution:

1. Stop all K⁺ supplementation immediately
2. Continuous ECG monitoring — look for peaked T waves, widened QRS
3. If ECG changes present → standard hyperK management: IV calcium gluconate (membrane stabiliser), insulin-dextrose, inhaled β₂-agonist ^[22]
4. Usually self-resolves within hours as the kidneys excrete the excess

• Check compliance with ATD
• Reassess TFT — is the patient truly euthyroid?
• If still thyrotoxic → dose adjustment or change of ATD
• Occurs during hyperthyroidism, not when euthyroid ^[6][13] — if the patient is genuinely euthyroid, reconsider the diagnosis (could have concurrent familial HypoPP)

Treatment of Gitelman ^[8]: Potassium and magnesium supplementation to normalise blood levels is the mainstay. Aldosterone antagonists (spironolactone or eplerenone) or ENaC blockers (amiloride) to decrease urinary K⁺ wasting.

Treatment of Bartter ^[21]: Sodium, chloride, potassium supplementation; spironolactone; NSAIDs (e.g., indomethacin — reduce GFR and therefore reduce K⁺ loss; but may cause gastric irritation and should be administered alongside acid suppression); surveillance renal US for nephrocalcinosis.

Risk of cardiac arrhythmia ^[6][13] Cardiac arrhythmia, particularly when [K⁺] < 2.0 ^[10]

Why does hypokalemia cause arrhythmia — from first principles?

Normal cardiac repolarisation depends on outward K⁺ currents (IKr, IKs) through K⁺ channels. When extracellular K⁺ drops, these channels paradoxically conduct less current (due to reduced K⁺ availability and channel rectification properties). The result:

1. Repolarisation is delayed → prolonged QT interval ^[10]
2. During the prolonged plateau phase, L-type Ca²⁺ channels may reactivate → early afterdepolarisation (EAD)
3. EADs can trigger ectopic beats → if these occur on the background of a heterogeneously prolonged action potential, a re-entrant circuit forms → TdP
4. TdP can self-terminate or degenerate into VF → sudden cardiac death

ECG changes — large U wave, loss of T wave, prolonged QT interval ^[10]. These ECG changes are the visible footprint of the repolarisation abnormality described above.

Rhabdomyolysis ^[10]

Why does the rhabdomyolysis from hypoK cause a paradox? The muscle cells lyse and dump their intracellular K⁺ into the extracellular space. So you can have rhabdomyolysis-induced hyperkalemia superimposed on the original hypokalemia — this makes electrolyte management especially complex.

Seldom respiratory muscles ^[6][13] Respiratory failure ^[23]

Ileus, constipation ^[10]

These GI complications are more commonly seen in chronic hypokalemia (diuretic use, hyperaldosteronism) than in the acute, self-limiting attacks of periodic paralysis, but can occur during prolonged severe attacks.

Polyuria ^[10]

May develop progressive proximal myopathy > 50y ^[3]

Why does this matter clinically? A 55-year-old patient with a history of HypoPP in adolescence who now has fixed proximal weakness is NOT having periodic paralysis attacks — they have developed a permanent vacuolar myopathy. The management is supportive (physiotherapy, occupational therapy) as the damage cannot be reversed.

• Unpredictable episodes of paralysis → significant anxiety, depression, and reduced quality of life
• Fear of public attacks → social avoidance
• Occupational limitations (inability to perform physically demanding jobs, risk during driving)
• Important to address with counselling, trigger avoidance education, and reliable prophylaxis