Thyroid and antithyroid drugs
Thyroid and antithyroid drugs function to correct thyroid hor-mone deficiency (hypothyroidism) and thyroid hormone excess (hyperthyroidism).
Thyroid drugs can be natural or synthetic hormones and maycontain triiodothyronine (T3), thyroxine (T4), or both.
Natural thyroid drugs are made from animal thyroid and include:
· thyroid USP (desiccated), which contains both T3 and T4
· thyroglobulin, which also contains both T3 and T4.
Synthetic thyroid drugs are actually the sodium salts of the Lisomers of the hormones. These synthetic hormones include:
· levothyroxine sodium, which contains T4
· liothyronine sodium, which contains T3
· liotrix, which contains both T3 and T4.
Thyroid hormones are absorbed variably from the GI tract, distrib-uted in plasma, and bound to serum proteins.
Thyroid drugs are metabolized through deiodination, primarily in the liver, and excreted unchanged in stool.
The principal pharmacologic effect is an increased metabolic rate in body tissues. Thyroid hormones affect protein and carbohy-drate metabolism and stimulate protein synthesis. They promote gluconeogenesis (the formation of glucose from free fatty acids and proteins) and increase the use of glycogen stores.
Thyroid hormones increase heart rate and cardiac output (the amount of blood pumped by the heart each minute). They may even increase the heart’s sensitivity to catecholamines and in-crease the number of beta-adrenergic receptors in the heart (stimulation of beta receptors in the heart increases heart rate and contractility).
Thyroid hormones may increase blood flow to the kidneys and increase the glomerular filtration rate (the amount of plasma filtered through the kidney each minute) in hypothy-roid patients, producing diuresis.
Thyroid drugs act as replacement or substitute hormones in these situations:
§ to treat the many forms of hypothyroidism
§ with antithyroid drugs to prevent goiter formation (an enlarged thyroid gland) and hypothyroidism
§ to differentiate between primary and secondary hypothyroidism during diagnostic testing
§ to treat papillary or follicular thyroid carcinoma.
Levothyroxine is the drug of choice for thyroid hormone replace-ment and thyroid-stimulating hormone suppression therapy.
Thyroid drugs interact with several common medications. (See Adverse reactions to thyroid drugs.)
· They increase the effects of oral anticoagulants, increasing the tendency to bleed.
· Cholestyramine and colestipol reduce the absorption of thyroid hormones.
· Phenytoin may displace thyroxine from plasma-binding sites, temporarily increasing levels of free thyroxine.
· Taking thyroid drugs with digoxin may reduce serum digoxin levels, increasing the risk of arrhythmias or heart failure.
· Carbamazepine, phenytoin, phenobarbital, and rifampin in-crease metabolism of thyroid hormones, reducing their effective-ness.
· Serum theophylline levels may increase when theophylline is administered with thyroid drugs.