What is the thyroid?
The thyroid is a small, butterfly-shaped gland located at the base of the throat. This gland plays a very important role in controlling the body's metabolism. It does this by producing thyroid hormones, primarily thyroxine (T4) and triiodothyronine (T3), substances 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 produce 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 elimination from the kidneys.
The body has an elaborate feedback system to control the amount of T4 and T3 in the blood. When blood levels of the hormones decrease, the hypothalamus releases thyrotropin-releasing hormone, which in turn causes the pituitary gland to release thyroid-stimulating hormone (TSH). TSH stimulates the thyroid gland to produce and secrete T4 (primarily) and T3. When the system is functioning normally, thyroid production turns on and off to maintain relatively stable levels of thyroid hormones.
Inside the thyroid, most of the T4 is stored bound to a protein called thyroglobulin. When the need arises, the thyroid gland produces more T4 and/or releases some of what is stored. In the blood, most T4 and T3 are bound to a protein called thyroxine-binding globulin (TBG) and are relatively inactive. The small amounts that are unbound, called free T4 or free T3, are the active forms of the hormone. T4 is converted to T3 in the liver and other tissues. T3 is primarily responsible for controlling the rate of body functions.