Thyroid Hormones

Through the secretion of thyroxine, T4, and its active derivative, T3, the thyroid governs myriad physiological events which bring about functional alterations in virtually all metabolic pathways and organs. Thyroid hormone modulates oxygen consumption and basal metabolic rate (BMR), as well as lipid, carbohydrate, and protein metabolism. It has fundamental effects on the nervous system both during development and in childhood and adulthood. Thyroid hormone regulates the synthesis and degradation rates of numerous proteins, including other growth factors and hormones, so that many of its effects emerge as secondary influences on other endocrine pathways. The effects of thyroid hormone fall into two categories of biological responses: (a) effects on cellular differentiation and development, particularly, but not limited to, the nervous system; and (b) effects on metabolic pathways through which the body uses carbohydrates, lipids, and proteins. These two actions are interconnected in that changes in development and growth are both a cause and a consequence of hormonal modulation of metabolism. This chapter covers thyroid hormone secretion, its regulation, and its actions and some of their mechanisms, setting the stage to observe the results of excess or insufficiency of this hormone. The symptoms of the most common disease states of the thyroid gland, covered here, are a direct consequence of an interruption of the known actions of its hormones. Furthermore, treatments of these conditions are based directly on our understanding of the mechanisms of the synthesis of thyroid hormone and its regulation.