Thiazide diuretics are an FDA-approved class of drugs that inhibit reabsorption of 3% to 5% of luminal sodium in the distal convoluted tubule of the nephron. By doing so, thiazide diuretics promote natriuresis and diuresis. Three thiazide diuretics are commonly used: hydrochlorothiazide (HCTZ), chlorthalidone, and indapamide.
HCTZ and chlorthalidone are FDA-approved for clinical use in the management of primary hypertension. These agents are used as either the sole modality of treatment or in conjunction with other antihypertensive drugs to effectively increase therapeutic response in severely hypertensive patients.
HCTZ and chlorthalidone are also FDA-approved for clinical use in adjunctive therapy in edema that is associated with chronic heart failure (CHF), hepatic cirrhosis, corticosteroids, and estrogen therapy. These agents are also used to manage edema caused by various forms of renal dysfunction such as nephrotic syndrome, acute glomerulonephritis, and chronic renal failure.
Indapamide is FDA-approved for the treatment of salt and fluid retention associated with CHF, and in the management of primary hypertension as either the sole treatment or in conjunction with other antihypertensive drugs.
Thiazide diuretics exert their diuretic effect via blockage of the sodium-chloride (Na/Cl) channel in the proximal segment of the distal convoluted tubule (DCT). When the Na/Cl channel is blocked, decreased levels of sodium cross the luminal membrane, thus decreasing the action of the sodium-potassium (Na/K) pump and decreasing Na and water passage to the interstitium.
The method of activation of thiazide diuretics is to cause a change in Na concentration distal to the DCT. Subsequently, ionic channels and pumps work to balance Na levels that have been disrupted. This secondary change to balance Na levels produces many adverse effects. The MOA and its effects on the nephron will be more thoroughly discussed below and will be referred to in the adverse effects section.
Blockage of the Na/Cl channel causes an increase in sodium and water retention in the lumen and a decrease in Na in the DCT. At the same time, blockage of the Na/Cl channel increases the flow of ions through the Na/Ca channel resulting in increased calcium reabsorption into the interstitium in exchange for Na return to the DCT.
Inhibition of the Na/Cl channel in the proximal segment of the distal convoluted tubule results in an increased delivery of sodium to the distal segment of the distal convoluted tubule and collecting tubule. This increase in Na increase causes the aldosterone-sensitive Na/K pump to increase sodium reabsorption in the principal cells. This exchange increases Na transfer into the interstitium and increases K transfer into the collecting tubules and lumen. This loss of K then causes intercalated cells in the collecting tubule (CT) to increase K reabsorption via the K/H+ pump and is also aldosterone-mediated.
Through aldosterone-mediated sodium retention that is precipitated by an increased Na flow to the CT, the CT will exhibit increase reabsorption of Na and excretion of both K and H ions into the urine.
Thiazide diuretics are administered orally as tablets. They should be taken in the morning with food. HCTZ and chlorthalidone have different dosing requirements for their indicated FDA uses listed above. Generally, for hypertension treatment, both drugs require a lower dosage starting at 25 mg daily and may be increased to 50 mg or 100 mg, respectively. The dosage should be increased based on the individual therapeutic needs of the patient. For patients suffering from fluid buildup and edema, dosing is started at 50 mg to 100 mg and 50 mg to 200 mg, respectively.
Adverse effects of thiazide diuretics stem from the ionic imbalance caused due to the initial Na loss in the DCT.
Adverse effects of taking these medications also include a possible increased risk of developing acute pancreatitis. It is hypothesized that thiazides have a toxic effect on the pancreas and cause increased pancreatic secretions and pancreatic ischemia. If symptoms of acute pancreatitis are observed the patient must immediately stop use of the thiazide diuretic and they should not be re-prescribed this medication by their physician.
Thiazide diuretics are contraindicated for use in patients with anuria and sulfonamide allergies.
An Evidence-based Approach to Thiazide Diuretics
Besides physicians, the use of thiazide diuretics is also monitored by pharmacists and nurses. In most cases, thiazide is prescribed for long periods, and hence, patient electrolyte levels need to be monitored. In addition, polypharmacy must be avoided, especially in the elderly. Patients need to be educated on the adverse effects of these agents and their presentation. For those patients with heart problems and susceptibility to arrhythmias, the levels of potassium must be closely followed. Also, the patient needs to be weighed at each visit to ensure that the fluid overload is resolving. Patients prone to hypokalemia must be educated on the consumption of foods that are rich in potassium. Finally, the patient's lipid levels need to be followed as thiazides have been known to cause hyperlipidemia. (Level V)
Thiazide diuretics have been in use for over half a century and are relatively safe and effective drugs for the treatment of hypertension and heart failure. These drugs are used in outpatients to manage chronic edema states. The majority of patients do respond to these agents, and the most common adverse effect reported is hypokalemia. To date, only a few reports of arrhythmias have been reported in patients taking thiazides. (Level V)
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