Thyroxine is an iodine-containing, thyroid hormone produced by the thyroid gland. Along with thyroxine, triiodothyronine is also produced in the thyroid gland. Thyroxine usually is converted into triiodothyronine by enzymes in target cells called deiodinases. Concentrations of thyroxine are usually greater in the body, but triiodothyronine is considered the major thyroid hormone. Thyroxine production is controlled by the pituitary gland. The hypothalamus will secrete thyrotropin-releasing hormones that stimulate the pituitary gland to secrete thyroid-stimulating hormone (TSH). TSH fuels the production of the thyroid hormones, thyroxine and triiodothyronine. 1 TSH will be triggered to release into the blood any time thyroxine levels are low.
