Thyroid and Antithyroid Drugs

Three hormones, thyroxine (3,5,3 ,5 -tetraiodothyronine, or T4), triiodothyronine (3,5,3 -triiodothyronine, or T3), and calcitonin (see Chapter 66) are secreted by the thyroid gland. The hormones T4 and T3 are iodinecontaining amino acid derivatives and are unique in that they have no discrete target tissue. Every tissue in the body is affected in some way by thyroid hormones, and almost all cells appear to require constant optimal amounts for normal operation. Thyroid hormones exert a wide variety of physiological actions through genomic and nongenomic mechanisms and influence the metabolism of proteins, carbohydrates, and lipids; cell morphology; membrane transport; ion homeostasis; oxygen consumption; heat production; and so on. Relatively constant circulating concentrations of T4 and T3 are required for normal growth and development and the proper functioning of the neural, reproductive, cardiovascular, gastrointestinal, and hematopoietic systems. Unlike most other hormones, whose circulating concentrations vary widely in response to external and internal stimuli, the circulating concentrations of thyroid hormones are usually held relatively constant over time. In health, two negative feedback control systems operate to maintain circulating thyroid hormone levels. The first, the hypothalamic–pituitary-thyroid axis (HPTA), acts to regulate the concentration of thyroid hormones in the blood by controlling their synthesis and secretion by the thyroid gland. The second negative feedback control system is the thyroid autoregulatory system. It is intrinsic to the thyroid gland and acts to ensure that an adequate supply of iodide is extracted from the blood and made available for thyroid hormone synthesis despite variations in dietary iodine intake. Worldwide, the most common thyroid disorder is hypothyroidism resulting from dietary iodine deficiency. In iodine-replete areas of the world, most thyroid disorders are the result of autoimmune disease. The symptoms manifested in hypothyroid and hyperthyroid states are largely independent of any underlying disorder of the thyroid gland itself; they are a function of the degree of hormone deficiency or excess. D R U G L I S T Thyroid and Antithyroid Drugs