What is the thyroid?
The thyroid is a small, butterfly-shaped gland located just below the Adam's apple. This gland plays a very important role in controlling the body's metabolism, that is, the rate at which the body uses energy. It does this by producing thyroid hormones (primarily thyroxine, or T4, and triiodothyronine, or T3), chemicals that travel through the blood to every part of the body. These thyroid hormones tell the cells in the body how fast to use energy and create proteins. The thyroid gland also makes calcitonin, a hormone that helps to regulate calcium levels in the blood by inhibiting the breakdown (reabsorption) of bone and increasing calcium excretion from the kidneys.
The body has an elaborate feedback system to control the amount of T4 and T3 in the blood. When blood levels decrease, the hypothalamus releases thyrotopin-releasing hormone, which in turn causes the pituitary gland to release thyroid-stimulating hormone (TSH). TSH stimulates the thyroid gland to produce and secrete thyroid hormones. When there is sufficient thyroid hormone in the blood, the amount of TSH decreases to maintain constant amounts of thyroid hormones, T4 and T3.
Inside the thyroid, most of the T4 is stored bound to a protein called thyroglobulin. When the need arises, the thyroid gland creates more T4 and/or releases some of what is stored. In the bloodstream, most T4 is bound to a protein called thyroxine-binding globulin (TBG) and is relatively inactive. T4 is converted to T3 by the liver and in many other tissues. T3 is primarily responsible for controlling the rate of body functions.