The thyroid is a small gland located below the Adam's apple in your neck. It releases hormones, thyroxine (T4) and triiodothyronine (T3), which increase the amount of oxygen your body uses and stimulate your cells to produce new proteins. By controlling the release of these hormones, the thyroid determines the metabolic rate of most of your body's organs.
The thyroid gland is regulated by thyroid-stimulating hormone (TSH), which is made by the pituitary gland in the brain. Normally, when thyroid hormone levels in the body are high, they will "switch off" the production of TSH, which in turn stops the thyroid from making more T4 and T3.
Problems occur when the thyroid gland becomes either underactive (hypothyroidism) or overactive (hyperthyroidism). Thyroid problems are more common in women than men. Cancer may also develop in the thyroid gland.