Thyroid Replacement Hormones

What are thyroid replacement hormones and how do they work?

Thyroid replacement hormones are medications used to treat hypothyroidism, a condition in which the production of thyroid hormone in the body is abnormally low. Thyroid hormones increase cellular metabolism (activity of cells) that is responsible for growth, development of tissues, maintenance of brain function, body temperature regulation and several other cellular processes. Low levels of thyroid hormones in the body can result in many problems given the numerous activities that they mediate. The thyroid gland, a gland found in the lower neck is responsible for the production of thyroid hormones. It produces two main hormones, thyroxine (T4) and triiodothyronine (T3). The hormone responsible for most of the biological effects in the body is T3. When T4 is released into the blood by the thyroid gland, most of it is converted to T3 which is responsible for the cellular metabolic processes. Commercially available thyroid hormones are either natural or synthetic (man-made). Desiccated thyroid or thyroid extract (Armor Thyroid, Nature-Throid), a natural thyroid hormone is derived from beef or pork. Levothyroxine sodium (for example, Synthroid, Levoxyl and Levothroid), is the synthetic version of thyroxine (T4), liothyronine sodium (Cytomel, Triostat), is the synthetic version of tT3 and liotrix (Thyrolar) is a synthetic thyroid hormone containing a mixture of T4and T3.

What are the side effects of thyroid replacement hormones?

Thyroid replacement hormones usually are well tolerated. Symptoms that occur during treatment are often due to toxic, elevated levels of thyroid hormones and resulting symptoms from hyperthyroidism. Symptoms may include chest pain, increased heart rate or pulse rate, excessive sweating, heat intolerance, nervousness, headache, insomnia, diarrhea, vomiting, weight loss, and fever. Some women may experience irregular menstrual cycles.

What are some examples of thyroid replacement hormones?

The following is a list of the thyroid replacement hormones that are available in the United States:

• levothyroxine sodium (Levothroid, Levoxyl, Synthroid, Tirosint, Unithroid)
• liothyronine sodium (Cytomel, Triostat)
• liotrix (Thyrolar)

For what conditions are thyroid replacement hormones used?

Thyroid replacement hormones are used to treat hypothyroidism (low production of thyroid hormone) and myxedema, a condition that is caused by prolonged hypothyroidism. Thyroid replacement hormones prevent thyroid hormone release from cancerous thyroid nodules and are used therefore to treat thyroid cancers. They also are used to manage thyrotoxicosis, a condition in which there are high levels of thyroid hormones resulting from over-active thyroid glands and too much thyroid hormone. Thyrotoxicosis may progress to hypothyroidism or cause the growth of goiters necessitating the use of thyroid replacement hormones.

Are there any differences among the different types of thyroid replacement hormones?

There is conflicting evidence regarding which hormone replacement therapy should be preferred. The American Association of Clinical Endocrinologists recommends that clinical hypothyroidism is best treated with synthetic T4 levothyroxine (for example levothyroxine and sodium [Synthroid, Levoxyl and Levothroid]). There is variability between the absorption and distribution of generic T4 compared to brand name preparations. Hence it is recommended that patients remain with specific brand names during treatment. There is also variability between generic formulations and brand names of pure T3 (liothyronine [Cytomel, Triostat]), combined T4/T3 formulations (liotrix [Thyrolar]) and thyroid extracts from animal sources (Armour Thyroid, Nature-Throid etc.). Thyroid extracts from animal sources are no longer available in the United States. Emerging information shows that combination of T4/T3 therapy may have some advantages over T4 in cognitive performance and mood but studies are not conclusive.

With which drugs do thyroid replacement hormones interact?

Thyroid replacement hormones should be used cautiously in people with diabetes since starting or discontinuing therapy may lead to a loss of control of the blood sugar requiring adjustments in doses of insulin or oral antidiabetic drugs (for example, glyburide [Micronase]). The effects of blood thinners such as warfarin (Coumadin) may be increased by thyroid replacement hormones warranting a decrease in the dose of warfarin in addition to monitoring of blood clotting.

Intravenous epinephrine administration in patients with coronary artery disease who are taking thyroid replacement hormones may increase the risk of complications such as difficulty in breathing and possibly heart attacks. The effectiveness of some beta blockers [for example, metoprolol (Lopressor) orpropranolol (Inderal)] may be reduced when a patient is converted from a state of hypothyroidism (under activity) to a normal state (euthyroid state). It also may be necessary to modify the dose of digoxin (Lanoxin) and theophylline (Slo-Bid) when a patient is converted from hypothyroidism (under activity) to a normal state (euthyroid state). There is increased elimination of theophylline in a euthyroid state compared to a state of hypothyroidism.

The effectiveness of thyroid replacement hormones may be decreased when given with drugs such as calcium carbonate, ferrous sulphate, cholestyramine (Questran) and colestipol (Colestid) that binds thyroid replacement hormones and prevent their absorption. This interaction may be reduced by separating the administration of these drugs from thyroid replacement hormones by four hours.