What is it? The thyroid is a small, butterfly-shaped gland just below the Adam's apple. It plays a very important role in controlling your body's
metabolism, that is, how your body uses energy. The thyroid controls energy use by producing hormones (mainly thyroxine, or
T4, and triiodothyronine, or
T3). Hormones are chemicals that travel through your blood to every part of your body. Thyroid hormones tell the cells in your body how fast to work. The thyroid also makes a hormone called
calcitonin.
The thyroid works with several other areas of the body called the pituitary gland and the hypothalamus. When you have enough thyroid hormone in your blood, the pituitary gland and hypothalamus tell the thyroid to stop making the hormone. When the body needs more thyroid hormones the hypothalamus passes a message through the pituitary to make the thyroid start producing hormones again.
The hypothalamus and pituitary gland are found at the base of the brain. The hypothalamus produces thyrotropin-releasing hormone (TRH). This acts on the pituitary which produces thyroid-stimulating hormone (TSH). TSH tells the thyroid to produce more thyroid hormones (T4 & T3).
Inside the thyroid, most of the T4 is stored attached to a protein called thyroglobulin. When more thyroid hormones are needed the thyroid makes more T4 and releases some of what is stored. In the bloodstream, most T4 is attached to a protein called thyroxine-binding globulin (TBG). When attached to this protein it has little effect but can be converted to T3 by the liver and many other tissues. T3 is most important in controlling the body’s energy use.
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).
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