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
thyroid USP (desiccated), which contains both T3 and T4
also contains both T3 and T4.
Synthetic thyroid drugs are actually the sodium
salts of the Lisomers of the hormones. These synthetic hormones include:
which contains T4
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
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
Levothyroxine is the drug of choice for
thyroid hormone replace-ment and thyroid-stimulating hormone suppression
Thyroid drugs interact with several common
medications. (See Adverse reactions to
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.