# Situation: A 64-year-old man with chronic atrial fibrillation and hypertension takes digoxin 0.125 mg and hydrochlorothiazide 25 mg orally every morning. He has no heart failure or kidney disease. He is admitted with generalized weakness, leg cramps, and palpitations. His serum potassium is 2.9 mEq/L (3.5–5.0 mEq/L). The hydrochlorothiazide is held, and he has a peripheral intravenous (IV) catheter in his forearm. His cardiac monitor shows flattened T waves and prominent U waves. Which explanation of how hydrochlorothiazide lowered his potassium is correct?

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## 문제

Situation: A 64-year-old man with chronic atrial fibrillation and hypertension takes digoxin 0.125 mg and hydrochlorothiazide 25 mg orally every morning. He has no heart failure or kidney disease. He is admitted with generalized weakness, leg cramps, and palpitations. His serum potassium is 2.9 mEq/L (3.5–5.0 mEq/L). The hydrochlorothiazide is held, and he has a peripheral intravenous (IV) catheter in his forearm.

His cardiac monitor shows flattened T waves and prominent U waves. Which explanation of how hydrochlorothiazide lowered his potassium is correct?

## 보기

1. It blocks potassium reabsorption in the thick ascending limb of the loop of Henle
2. It shifts potassium out of the blood and into cells, as insulin does
3. It blocks aldosterone, so the collecting duct excretes more potassium
4. More sodium reaches the collecting duct, where it is exchanged for potassium **✔ 정답**

**정답: 4**

## 해설

Thiazides block sodium–chloride reabsorption in the distal convoluted tubule, so more sodium reaches the collecting duct. There it is reabsorbed in exchange for potassium (and hydrogen ions), which are lost in the urine. The flattened T waves and U waves reflect the resulting hypokalemia.

## 심화 해설

Mechanism of thiazide-induced hypokalemia

Thiazide diuretics such as hydrochlorothiazide act primarily on the distal convoluted tubule (DCT), where they inhibit the Na⁺–Cl⁻ cotransporter (NCC). This reduces sodium and chloride reabsorption at that site. As a result, a larger amount of sodium remains in the tubular fluid and travels downstream to the collecting duct.

In the collecting duct, sodium is reabsorbed through epithelial sodium channels (ENaC) in principal cells. This sodium entry creates a negative electrical gradient in the tubular lumen, which drives the secretion of potassium through renal outer medullary potassium (ROMK) channels and, under high-flow conditions, large-conductance potassium (BK) channels [1]. The more sodium delivered to the collecting duct, the greater the potassium secretion and urinary potassium loss. This explains why thiazide use produces hypokalemia.

The option that best captures this sequence is: more sodium reaches the collecting duct, where it is exchanged for potassium. The exchange is not a direct one-for-one molecular swap; rather, sodium reabsorption generates the electrochemical driving force for potassium secretion. The net clinical effect is potassium wasting.

Watch out! Thiazides do not act on the thick ascending limb of the loop of Henle. That is the site of loop diuretics such as furosemide, which inhibit the Na⁺–K⁺–2Cl⁻ cotransporter (NKCC2). Thiazides also do not shift potassium into cells like insulin or beta-agonists do, and they do not block aldosterone. In fact, the volume contraction and sodium loss caused by thiazides can activate the renin–angiotensin–aldosterone system, which would tend to increase potassium excretion further.

The ECG findings in this patient are consistent with hypokalemia. Flattened or inverted T waves and prominent U waves reflect delayed ventricular repolarization. In severe cases, the T and U waves may fuse, producing a pseudo-prolonged QT interval, as described in a case of thiazide-induced hypokalemic paralysis [2]. The potassium value of 2.9 mEq/L is below the reference range of 3.5–5.0 mEq/L and places the patient at risk for both muscle weakness and cardiac dysrhythmias.

| Distractor | Why it is incorrect |
| --- | --- |
| Blocks potassium reabsorption in the thick ascending limb | That describes loop diuretics, not thiazides. Thiazides act on the distal convoluted tubule. |
| Shifts potassium into cells, as insulin does | Thiazides do not have a direct cellular shift effect. Insulin, beta-agonists, and alkalosis cause intracellular potassium shifts. |
| Blocks aldosterone, so the collecting duct excretes more potassium | Aldosterone antagonists reduce potassium excretion. Thiazides do not block aldosterone; they may indirectly increase aldosterone activity. |
| More sodium reaches the collecting duct, where it is exchanged for potassium | Correct. Increased distal sodium delivery enhances potassium secretion in the collecting duct. |

Key point! For a patient taking digoxin, hypokalemia is especially dangerous because low extracellular potassium increases digoxin binding to the Na⁺/K⁺-ATPase pump, potentiating digoxin toxicity. This patient’s palpitations and ECG changes should prompt immediate potassium repletion and close cardiac monitoring.

The role of ROMK endocytosis in potassium conservation is relevant to understanding why the kidney normally prevents excessive potassium loss during dietary deficiency [1]. However, when thiazides increase distal sodium delivery, the electrochemical gradient favoring potassium secretion overrides these conservation mechanisms, leading to urinary potassium wasting.References (research sources)

- [1]The endocytic adaptor ARH facilitates potassium conservation by regulating ROMK and BK.Research articleAl-Qusairi L, Zapf AM, Li D, Woodward OM, Welling PA. (2025) · DOI: 10.1152/ajprenal.00248.2025

- [2]A Case of Thiazide-induced Hypokalemic Paralysis.Case reportSchell E, Pathman J, Pescatore R, Bianchi PW. (2019) · DOI: 10.5811/cpcem.2019.3.42062

## 임상 시나리오

Thiazide-Induced Hypokalemia: Clinical Recognition and ManagementUnderstanding the mechanism of potassium wasting in patients on hydrochlorothiazide
Thiazides like hydrochlorothiazide inhibit the Na⁺–Cl⁻ cotransporter (NCC) in the distal convoluted tubule, increasing sodium delivery to the collecting duct. This enhanced sodium load drives potassium secretion through ROMK channels, resulting in urinary potassium loss and hypokalemia.

A serum potassium of 2.9 mEq/L (normal 3.5–5.0 mEq/L) with flattened T waves and prominent U waves on ECG confirms clinically significant hypokalemia. Patients may present with generalized weakness, leg cramps, and palpitations.

CautionDo not confuse thiazide action with loop diuretics (which target the thick ascending limb) or potassium-sparing diuretics (which block aldosterone). Thiazides do not shift potassium intracellularly like insulin; they cause renal potassium wasting through increased distal sodium delivery.

## 핵심 개념

- **Thiazide diuretics** — Antihypertensive drugs that inhibit the Na+-Cl- cotransporter in the distal convoluted tubule, leading to increased sodium delivery to the collecting duct and subsequent potassium wasting.
- **Hypokalemia** — A serum potassium level below 3.5 mEq/L, often presenting with muscle weakness, leg cramps, and characteristic ECG changes such as flattened T waves and prominent U waves.
- **Collecting duct** — The final segment of the nephron where sodium reabsorption via ENaC channels creates an electrochemical gradient that drives potassium secretion through ROMK channels.
- **Na+-Cl- cotransporter (NCC)** — A transport protein in the distal convoluted tubule that is the primary target of thiazide diuretics, responsible for sodium and chloride reabsorption.

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