Measurements of Thyroxine and Triiodothyronine

Abstract

Thyroid hormone is a potent regulator of cellular proliferation and metabolic rate and must be maintained within an optimal range for normal development and health. The severe growth and neurologic injuries observed in children with untreated congenital hypothyroidism illustrate this. Thyroid hormone metabolism, which describes the biochemical activation and inactivation of thyroid hormones, is a powerful mechanism regulating thyroid hormone action. In this chapter, we will review the glandular synthesis of thyroid hormone and then discuss the metabolic pathways responsible for its activation and inactivation in peripheral tissues, focus- ing on deiodination as the dominant pathway in postnatal vertebrates and the most clinically relevant for thyroid function testing in patients. Subsequent chapters will discuss other sites of regulation, including the genes regulating thyroid hormone production and action (Chap. 2), availability of iodine (Chap. 3), and thyrotropin regulation