Comparative pharmacology of the thyroid hormones.

The thyroid hormones tetraiodothyronine and triiodothyronine (T3) are involved in a variety of biologic mechanisms. Thyroid hormones regulate protein, lipid, and carbohydrate anabolism and improve oxygen consumption in almost all body tissues. Thyroid hormone is formed by coupling of iodinated tyrosine molecules in the thyroid gland. Tetraiodothyronine is converted to T3, the active thyroid hormone. Drugs, disease states that induce severe physiologic stress, and surgical conditions (eg, cardiopulmonary bypass) can alter the levels of circulating active hormone by interfering with the peripheral conversion of tetraiodothyronine to T3. This low T3 ("euthyroid sick syndrome") state is manifested by low circulating levels of T3 and hemodynamic abnormalities. Active thyroid hormone replacement with T3, because of its pharmacologic actions, may be an ideal therapy in these conditions.