How thiazides are tested on Step 1

Expect questions on calcium handling, the metabolic side-effect cluster, hyponatremia in older adults, or the use in nephrogenic diabetes insipidus.

workspace_premium The calcium paradox

Thiazides lower urinary calcium and can unmask hyperparathyroidism by raising serum calcium. Loop diuretics do the opposite.

Mechanism in the distal convoluted tubule

NCC reabsorbs about 5–10% of filtered sodium. Blocking it lowers intracellular Na⁺, which increases basolateral Na⁺/Ca²⁺ exchange and pulls more Ca²⁺ from the lumen through apical TRPV5 channels.

NCCblockedDistalconvolutedtubule↓ CellNa⁺Less Na⁺entryNa⁺/Ca²⁺exchangerBasolateral,works harder↑ Ca²⁺reabsorbedThrough TRPV5↓ UrineCa²⁺Fewer calciumstones
Thiazides retain calcium. Mild volume contraction also increases proximal calcium reabsorption.
ParameterEffect
Urine Na⁺, Cl⁻↑
Serum K⁺↓ (more distal Na⁺ delivery)
Serum Ca²⁺↑ (urine Ca²⁺ ↓)
Serum Na⁺↓ (impaired diluting ability; ADH effect)
Acid–baseMetabolic alkalosis
Glucose, lipids, uric acid↑

Thiazide-induced hyponatremia is most common in older women and can be severe.

Clinical uses

Thiazides have uses beyond diuresis.

Thiazide usesHypertensionFirst-line;chlorthalidonepreferredCalciumstonesIdiopathichypercalciuriaNephrogenicDI↓ Urine volumeMild edemaWith normal GFROsteoporosisCalciumretention
Thiazide uses. In nephrogenic DI, volume contraction increases proximal sodium and water reabsorption, so less water reaches the ADH-unresponsive collecting duct.

Adverse effects

Thiazide toxicity is largely metabolic.

Adverse effectMechanism or note
HypokalemiaIncreased Na⁺ delivery to the collecting duct; aldosterone activation
HyponatremiaImpaired urine dilution in the distal tubule
HyperglycemiaHypokalemia reduces insulin release
HyperlipidemiaMild increases in LDL and triglycerides
Hyperuricemia, goutCompetes with urate for secretion; volume contraction
HypercalcemiaIncreased distal calcium reabsorption
Sulfa allergyThiazides are sulfonamides
PhotosensitivityHydrochlorothiazide is linked to nonmelanoma skin cancer

Mnemonic: HyperGLUC — Glycemia, Lipidemia, Uricemia, Calcemia.

ThiazideUrine Ca²⁺ ↓Serum Ca²⁺ ↑Prevents calcium stonesCan unmask hyperparathyroidismContraindicated in hypercalcemiaLoopUrine Ca²⁺ ↑Serum Ca²⁺ ↓Promotes calcium stonesTreats hypercalcemiaUsed in hypercalcemic crisis with salinevs
Opposite calcium effects. Both cause hypokalemia, metabolic alkalosis, and hyperuricemia.

Clinical application

Thiazides often appear in vignettes about stones and diabetes insipidus.

functions Worked example — the lithium patient

A man with bipolar disorder on long-term lithium has polyuria and polydipsia. Urine osmolality stays low after water deprivation and does not rise after desmopressin. Which diuretic reduces his urine output, and what must be monitored?

  1. No response to desmopressin means nephrogenic diabetes insipidus — lithium enters principal cells through ENaC and impairs ADH signaling.
  2. Thiazides cause mild volume contraction, increasing proximal reabsorption so less water reaches the collecting duct.
  3. Amiloride blocks ENaC, reducing lithium entry into principal cells.
  4. Thiazides increase proximal lithium reabsorption, raising lithium levels.

Answer. Amiloride (first choice for lithium-induced nephrogenic DI) or a thiazide. If a thiazide is used, monitor lithium levels closely because it raises them.