Thyroid hormone is either of the tyrosine-based hormones, thyroxine (T₄) or triiodothyronine (T₃), which are secreted by the follicular cells of the thyroid gland. These iodine-containing hormones, which are formed in the thyroid from thyroglobulin and then released into the blood stream, help control the rate of all metabolic processes in the body and influence physical development and activity of the nervous system.

A normal thyroid gland produces about 20 times more T₄ than T₃. However, T₃ is three to four times more potent as a hormone than T₄. v When the level of thyroid hormones in the blood drops too low, the pituitary gland produces thyroid-stimulating hormone (TSH), which causes the thyroid gland to produce more hormones. Under the influence of TSH, the thyroid manufactures and secretes more T₃ and T₄, thereby raising their concentrations in the blood. The pituitary senses this and responds by lowering its production of TSH.