Thyroid drugs

Thyroid drugs

Thyroid drugs can be natural or synthetic hormones and may contain triiodothyronine (T₃), thyroxine (T₄), or both.

All natural

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.

Man-made

Synthetic thyroid drugs are actually the sodium salts of the L-isomers of the hormones. These synthetic hormones include:

• levothyroxine sodium, which contains T₄
• liothyronine sodium, which contains T₃
• liotrix, which contains both T₃ and T₄.

Pharmacokinetics

Thyroid hormones are absorbed variably from the GI tract, distributed in plasma, and bound to serum proteins.

Metabolism and excretion

Thyroid drugs are metabolized through deiodination, primarily in the liver, and excreted unchanged in stool.

Pharmacodynamics

The principal pharmacologic effect is an increased metabolic rate in body tissues. Thyroid hormones affect protein and carbohydrate 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.

They get the heart pumping…

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 increase the number of beta-adrenergic receptors in the heart (stimulation of beta receptors in the heart increases heart rate and contractility).

…and the blood flowing

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 hypothyroid patients, producing diuresis.

Pharmacotherapeutics

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.

Warning!

Adverse reactions to thyroid drugs

Most adverse reactions to thyroid drugs result from toxicity.

GI impact

Adverse reactions in the GI system include:

• diarrhea
• abdominal cramps
• weight loss
• increased appetite.

Heart of the matter

Adverse reactions in the cardiovascular system include:

• palpitations
• sweating
• rapid heart rate
• increased blood pressure
• angina
• arrhythmias.

Toxic topics

General manifestations of toxic doses include:

• headache
• tremor
• insomnia
• nervousness
• fever
• heat intolerance
• menstrual irregularities.

The drug of choice

Levothyroxine is the drug of choice for thyroid hormone replacement and thyroid-stimulating hormone suppression therapy.

Drug interactions

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 increase metabolism of thyroid hormones, reducing their effectiveness.
• Serum theophylline levels may increase when theophylline is administered with thyroid drugs.