Functionally, the thyroid gland can be thought of as having 2 parts. The parafollicular (or C) cells secrete calcitonin and play a role in calcium homeostasis; they do not affect thyroid physiology. Thyroid follicles are made up of a single layer of epithelial cells surrounding colloid, which consists mostly of thyroglobulin, the storage form of the thyroid hormones T4 and T3.
T4 (thyroxine), the main secretory product of the thyroid gland, contains 4 iodine atoms. Deiodination of T4, which occurs mainly in the liver and kidneys, gives rise to T3 (triiodothyronine), the metabolically active form of thyroid hormone. Eighty percent of serum T3 is derived through deiodination; the remainder is secreted by the thyroid. Only a small fraction of these hormones circulates freely in the plasma (0.02% of total T4 and 0.30% of total T3); the remainder is bound to the proteins thyroxine-binding globulin (TBG), transthyretin, and albumin.
Thyroid function is regulated by the interrelationship of hypothalamic, pituitary, and thyroid activity. Thyrotropin-releasing hormone (TRH), which is secreted by the hypothalamus, causes the synthesis of thyrotropin (or thyroid-stimulating hormone, TSH) and its release from the anterior pituitary. TSH, in turn, stimulates the thyroid, leading to the release of T4 and T3. In this negative-feedback loop, increased levels of T4 and T3 inhibit the release of TSH and the TSH response to TRH at the level of the pituitary.
The main role of the thyroid hormones is regulation of tissue metabolism through their effects on protein synthesis. Normal development of the central nervous system requires adequate amounts of thyroid hormone during the first 2 years of life. Congenital hypothyroidism results in irreversible cognitive disabilities (cretinism). Normal growth and bone maturation also depend on sufficient hormone levels.
Excerpted from BCSC 2020-2021 series: Section 1 - Update on General Medicine. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.