regulation of thyroid hormone production, the actions of the thyroid hormones, the development of thyroid function during fetal life, and the effects of perchlorate when given deliberately to humans. The effects of environmental exposure to perchlorate are summarized in Chapter 3.
The thyroid is a butterfly-shaped gland in the front of the neck. It weighs about 1-1.5 g at birth and 10-20 g in healthy adults in the United States. It contains millions of spherical follicles, each composed of a single layer of cells known as thyroid follicular cells surrounding a space, or lumen, filled with fluid known as colloid (Figure 2-1). Thyroid follicles are the functional units of the thyroid gland. The colloid consists mostly of thyroglobulin, a thyroid protein that serves as the framework for production of T4 and T3 and as the storage form of the two hormones.
T4 and T3 are the only biologically active substances that contain iodine. They are similar in that each has two six-member rings—an inner and an outer ring (Figure 2-2)—connected by an ether linkage. Their inner rings have two iodine atoms; T4 has two iodine atoms in its outer ring, whereas T3 has only one. The compound formed if an iodine atom is removed from the inner ring of T4 is 3,3',5'-triiodothyronine (reverse T3), which has no biologic activity.
Iodine is an intrinsic component of T4 and T3. Transfer of iodide from the circulation into the thyroid gland is therefore an essential step in the synthesis of the two hormones.2 Iodide is transported from the bloodstream