Thyroxine (3,5,3‚Ä≤,5‚Ä≤-tetraiodothyronine) or T4 is one of two major hormones derived from the thyroid gland, the other being triiodothyronine (T3). The major form of thyroid hormone in the blood is thyroxine (T4), which has a longer half-life than T3. In humans, the ratio of T4 to T3 released into the blood is approximately 14:1. T4 is converted to the active T3 (three to four times more potent than T4) within cells by enzymes known as deiodinases (5‚Ä≤-iodinase). Thyroxine is synthesized via the iodination of tyrosines (monoiodotyrosine) and the coupling of iodotyrosines (diiodotyrosine) in the thyroglobulin. Iodine is critical to the synthesis of thyroxine and other thyroid hormones. Through a reaction with the enzyme thyroperoxidase, iodine is covalently bound to tyrosine residues found in the thyroglobulin protein, forming monoiodotyrosine (MIT) and diiodotyrosine (DIT). Linking two moieties of DIT produces thyroxine. Combining one molecule of MIT and one molecule of DIT produces triiodothyronine. Thyroxine is released from thyroglobulin by proteolysis and secreted into the blood. Iodide is actively absorbed from the bloodstream and concentrated in the thyroid follicles where thyroxine is produced. If there is a deficiency of dietary iodine, the thyroid enlarges in an attempt to trap more iodine, resulting in a condition called goitre. More specifically, the lack of thyroid hormones will lead to decreased negative feedback on the pituitary gland, leading to increased production of thyroid-stimulating hormone, which causes the thyroid to enlarge, leading to goitre. Thyroxine can be peripherally de-iodinated to form triiodothyronine which exerts a broad spectrum of stimulatory effects on cell metabolism.  Thyroid hormones function via a well-studied set of nuclear receptors, termed the thyroid hormone receptors. They act on nearly every cell in the body. In particular, thyroid hormones act to increase the basal metabolic rate, affect protein synthesis, help regulate long bone growth (synergy with growth hormone) and neural maturation, and increase the body's sensitivity to catecholamines (such as adrenaline) by permissiveness. The thyroid hormones are essential to proper development and differentiation of all cells of the human body. These hormones also regulate protein, fat, and carbohydrate metabolism, affecting how human cells use energetic compounds. They also stimulate vitamin metabolism. Numerous physiological and pathological stimuli influence thyroid hormone synthesis. Levothyroxine, a manufactured form of thyroxine, was the most prescribed medication in the United States with more than 114 million prescriptions.