What is it?
The thyroid is a small, butterfly-shaped gland just below the Adam's apple. It has two lobes that sit either side of the windpipe. It plays a very important role in controlling your body's metabolism, that is, how your body uses the energy contained within its cells and the rate at which it uses it. This is also called your metabolic rate. To do this the thyroid gland produces hormones. Hormones are chemicals that act as your body’s messengers and travel through your blood sending a signal to different parts of your body. The hormones that the thyroid produces act on the cells in other parts of the body to increase the rate at which they use energy. The thyroid gland produces 3 main hormones: Thyroxine (T4), triiodothyronine (T3) and calcitonin. The first two hormones affect your metabolism by increasing the metabolic rate (making you use more energy). T3 can be made in the thyroid from the breakdown of T4 and also by the same mechanism in other tissues in the body. The last hormone calcitonin is responsible for controlling the levels of calcium and phosphorus in the blood which we need to keep our bones strong and healthy. It does not affect our metabolic rate.
Metabolism is important as the body needs fuel (energy) to keep all its parts working on a daily basis, for example to keep our heart beating and allow our lungs to move so that we can breathe. The rate at which we use this energy controls how quickly the cells and the tissues in our body work. If our body parts work too quickly or too slowly we may start to feel unwell. Because of this the production of the hormones from the thyroid gland need to be controlled. The brain controls the production of these hormones from thyroid gland via two areas called the hypothalamus and the pituitary gland.
The hypothalamus first produces a chemical called thyrotropin-releasing hormone (TRH). This travels to the pituitary gland which sits at the base of the hypothalamus and tells it to produce a chemical called thyroid-stimulating hormone (TSH). TSH is then released by the pituitary gland into the blood stream and travels to the thyroid gland to tell it to produce more thyroid hormones (T4 & T3). When you have enough thyroid hormone in your blood, the pituitary gland and the hypothalamus tell the thyroid to stop making the hormone by reducing the production of TRH and TSH. When the body needs more thyroid hormones the hypothalamus and pituitary secrete TRH and TSH again so that the thyroid gland is told to make more thyroid hormones. In this way the levels of T4 and T3 in your body are tightly regulated and your metabolic rate is controlled at the correct level in order for your body to function properly.
Another way the thyroid gland makes sure that we have the correct level of thyroid hormone is to store some of it inside the gland. Inside the thyroid, most of the T4 is stored attached to a protein called thyroglobulin. When more thyroid hormones are needed the thyroid not only makes more T4 but releases some of what is stored into the bloodstream. Once in the bloodstream, most of the T4 is attached to a protein called thyroxine-binding globulin (TBG). The tissues cannot absorb the T4 as well when it is attached to this protein. However when the T4 is needed by the tissues it is released from the protein so that the body can use it easily. T4 can also be converted to the more active T3 by the liver and many other tissues when needed. Most UK laboratories now measure T4 and T3 hormones in their active ‘free’ state which means that they are not bound to proteins. These tests are usually referred to as Free T4 (FT4) and Free T3 (FT3).