Thiazide

Thiazide is a term used to describe a type of molecule^[1] and a class of diuretics^[2] often used to treat hypertension (high blood pressure) and edema (such as that caused by heart, liver, or kidney disease).

The thiazides and thiazide-like diuretics reduce the risk of death, stroke, heart attack and heart failure due to hypertension.^[3] In most countries, the thiazides are the cheapest antihypertensive drugs available.^[4]

Mechanism of action

The members of this class of diuretics are derived from benzothiadiazine. They control hypertension in part by inhibiting reabsorption of sodium (Na⁺) and chloride (Cl⁻) ions from the distal convoluted tubules in the kidneys by blocking the thiazide-sensitive Na⁺-Cl⁻ symporter.^[5] Thiazides also cause loss of potassium and an increase in serum uric acid. The term "thiazide" is also often used for drugs with a similar action that do not have the thiazide chemical structure, such as chlortalidone and metolazone. These agents are more properly termed thiazide-like diuretics.

Denomination

That thiazide refers to both the type of molecule and the medication can sometimes lead to confusion, because some molecules (thiazide-like diuretics) are often considered as thiazide diuretics, although they are not thiazides from a chemical perspective. In this context, "thiazide" is taken to refer to a drug which acts at a "thiazide receptor",^[6] which is believed to be a sodium-chloride symporter.

Primary uses

Thiazides are often used to treat hypertension, although they are also used to treat congestive heart failure and symptomatic edema. They are the recommended first-line treatment in the US (JNC VII)^[7] guidelines and the National Institute for Health and Clinical Excellence/British Hypertension Society guidelines^[8] and a recommended treatment in the European (ESC/ESH)^[9] guidelines. Whilst once regarded as the first choice of treatment for cases of uncomplicated hypertension when pharmacotherapy is indicated, they have been replaced by ACE inhibitors in Australia due to their propensity to increase risk of diabetes mellitus type 2.^[10] Low-dose Thiazides have been shown to reduce hypertension-related morbidity and mortality,^[3] although how they lower blood pressure in the long term is not fully understood. When administered acutely thiazides lower blood pressure by causing diuresis, a fall in plasma volume and a reduction in cardiac output. However, after chronic use thiazides cause a reduction in blood pressure by lowering peripheral resistance (i.e. vasodilation). The mechanism of this effect is uncertain but it may involve effects on 'whole body' or renal autoregulation, or direct vasodilator actions either through inhibition of carbonic anhydrase^[11] or by desensitizing the vascular smooth muscle cells to the rise in intracellular calcium induced by norepinephrine.^[12]

Other uses

Thiazides also lower urinary calcium excretion, making them useful in preventing calcium-containing kidney stones. This effect is associated with positive calcium balance and is associated with an increase in bone mineral density and reductions in fracture rates attributable to osteoporosis. By a lesser understood mechanism, thiazides directly stimulate osteoblast differentiation and bone mineral formation, further slowing the course of osteoporosis.^[13]

Because of their promotion of calcium retention, thiazides are used in the treatment of Dent's Disease or idiopathic hypercalciuria.

Thiazide may be combined with ACE inhibitors to increase diuresis without changing plasma potassium concentrations. While ACE inhibitors cause diuresis with potassium retention, thiazide increases potassium excretion. Their combined effects on potassium cancel each other out. Thiazides have no major effect on renal blood flow, but are likely to decrease glomerular filtration rate.

Breast milk

Thiazides pass through breast milk, and in some cases, decrease the flow of breast milk. There is no specific information regarding the use of thiazides in children, but it is still advised that mothers avoid using thiazides during the first month of breast feeding.

Contraindications

Thiazides reduce the clearance of uric acid and are therefore to be used with caution in patients with gout or hyperuricemia.^[14][15]

Thiazides can decrease placental perfusion and adversely affect the fetus so should be avoided in pregnancy.^[15][16]

Mechanisms of hypokalemia

There are several mechanisms by which thiazide diuretics cause hypokalemia (decreased plasma potassium concentration):

• Increased delivery of sodium to the collecting ducts causes increased cellular uptake of Na from the lumen by apical Epithelial Na Channels (ENaCs). This then causes the basolateral Na/K exchanger to more actively exchange Na for K, which is then passively secreted into the lumen through apical channels, resulting in K loss. (Moreover, the increased delivery of K to the collecting ducts facilitates the exchange of K for H by the H/K exchangers on the intercalated alpha cells, resulting in loss of H [metabolic alkalosis].)
• Activation of renin-angiotensin-aldosterone system by the diuretic hypovolemia: body responds to hypovolemia by opposing diuresis, one effect of which is to produce aldosterone which stimulates the Na/K exchanger, resulting in further loss of potassium. For this reason, ACE inhibitors, which inhibit angiotensin II production and therefore aldosterone activation, are frequently used in combination with thiazides to combat hypokalemia.
• Flow rate in nephron is increased under diuresis, reducing potassium concentration in the lumen, thus increasing the potassium gradient. Potassium loss through the many potassium channels, such as ROMK. These are not exchangers; they allow facilitated diffusion, so the increased gradient is directly responsible for increased diffusion.

Complications

• Hyperglycemia
• Hyperlipidemia
• Hyperuricemia
• Hypercalcemia
• Hypokalemia
• Hyponatremia

References

External links