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The thyroid gland produces the thyroid hormone triiodothyronine (T3) and thyroxine (T4). The thyroid gland produces more T4 than T3, and, in fact, most of the T3 in the body actually results from conversion of T4 to T3 outside the thyroid gland. T4 and T3 have the same effects on the body, and increasing doses of either cause the thyroid gland to make less thyroid hormones. (This is referred to as "feedback inhibition," in which the levels of a chemical in the body regulates its own production.) T3 is more potent that T4, which means that one mg of T3 has a greater effect on the body than one mg of T4.
Levothyroxine (Synthroid) is a synthetic version of T4, and liothyronine (Cytomel) is a synthetic version of T3. Both are used for the treatment of hypothyroidism, but levothyroxine is preferred because T3 is absorbed from the intestine very rapidly, and this may cause mild thyroid hormone toxicity (hyperthyroidism) in some patients. Also, the test that measures the level of T4 in the blood (TT4) is not useful when T3 is administered. (Specifically, if TT4 is used to monitor the adequacy of treatment when T3 is administered, it shows a low level of T4 which can lead to the erroneous decision to administer more T3.) Finally, since T4 is converted to T3 in the body, there is no advantage to administering T3 products.
Medical Author: Ruchi Mathur, M.D.