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Thiazides

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Pharmacology

Summary

Thiazides are a group of diuretics that primarily act on the distal convoluted tubule of the nephron. These drugs are especially beneficial in non-edematous states and are notably effective in managing hypertension. Contrary to loop diuretics, which increase the excretion of calcium, thiazides promote calcium reabsorption, a mechanism crucial in preventing the formation of renal calcium stones.

Approximately 10% of filtered sodium undergoes reabsorption in the distal convoluted tubule, where sodium chloride plays a pivotal role in further diluting tubular fluid. Thiazide diuretics inhibit sodium chloride reabsorption by targeting the sodium-chloride cotransporter on the apical membrane, leading to increased diuresis. Thiazides can elevate serum levels of glucose, cholesterol, and calcium, while also potentially causing hyperuricemia and hyponatremia. Additionally, thiazides promote the excretion of potassium and hydrogen ions in DCT. This excretory effect can lead to hypokalemia and an associated increase in bicarbonate ions in the bloodstream, culminating in a condition called contraction alkalosis. Clinically, thiazides serve multiple purposes, from first-line treatments for hypertension to adjunctive therapy in heart failure and addressing nephrogenic diabetes insipidus.

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FAQs

How do thiazide diuretics act within the distal convoluted tubule (DCT)?

Thiazide diuretics target the distal convoluted tubule (DCT) in the kidneys by inhibiting the NaCl cotransporter on the apical membrane, decreasing sodium and chloride reabsorption. This action promotes natriuresis, as well as indirectly increasing the reabsorption of calcium via the Na+/Ca+ transporter located in the basolateral membrane.

What effects do thiazide diuretics have on calcium homeostasis?

Thiazide diuretics facilitate the reabsorption of calcium in the distal convoluted tubule, which decreases calcium excretion in the urine. Thiazide inhibition of the NaCl cotransporter results in reduced reabsorption of sodium in the DCT, which indirectly increases the reabsorption of calcium via the Na+/Ca+ transporter. This enhanced calcium retention can benefit patients with osteoporosis by promoting calcium retention in bones. However, it can also lead to hypercalcemia, characterized by elevated calcium levels in the blood.

In which medical conditions are thiazide diuretics typically prescribed?

Thiazide diuretics are often prescribed as a first-line therapy for mild to moderate hypertension. While they are employed in the symptomatic management of heart failure, loop diuretics remain the primary choice. Thiazides are also effective in treating nephrogenic diabetes insipidus and in preventing calcium stone formation. Their ability to boost calcium reabsorption makes them a potential treatment option for osteoporosis.

What notable side effects are associated with thiazide diuretics?

Thiazide diuretics, while beneficial, can produce several side effects. These include hypercalcemia, hyperlipidemia,  hyperglycemia, and hyperuricemia, which can trigger gout. Additionally, they can elevate lithium levels, induce potassium depletion (hypokalemia), and cause  hyponatremia. It's also important to note that thiazides are sulfa-based drugs, and are contraindicated in patients with sulfa allergies.

How do thiazides contribute to contraction alkalosis?

Thiazides promote the excretion of potassium and hydrogen ions in the distal convoluted tubule. This excretory effect can lead to a decrease in serum potassium and an associated increase in bicarbonate ions in the bloodstream, culminating in a condition called contraction alkalosis.