endocrine and metabolic disorders
Overview of Thyroid Function
Introduction
The thyroid gland, located in the anterior neck just below the cricoid cartilage, consists of 2 lobes connected by an isthmus. Follicular cells in the gland produce the 2 main thyroid hormones: Tetraiodothyronine (thyroxine, T4) Triiodothyronine (T3) These hormones act on cells in virtually every body tissue by combining with nuclear receptors and altering expression of a wide range of gene products. Thyroid hormone is required for normal brain and somatic tissue development in the fetus and neonate, and, in people of all ages, regulates protein, carbohydrate, and fat metabolism. T3 is the most active form in binding to the nuclear receptor; T4 has only minimal hormonal activity. However, T4 is much longer lasting and can be converted to T3 (in most tissues) and thus serves as a reservoir or prohormone for T3. A 3rd form of thyroid hormone, reverse T3 (rT3), has no metabolic activity; levels of rT3 increase in certain diseases. , which is released in response to hypercalcemia and lowers serum calcium levels (see Regulation of Calcium Metabolism ). 