free thyroxine


thyroxine

 (T4) [thi-rok´sin] a thyroid hormone that contains iodine" >iodine and is a derivative of the amino acid tyrosine" >tyrosine, occurring naturally as l-thyroxine; its chemical name is tetraiodothyronine. It is formed and stored in the thyroid follicles as thyroglobulin and released from the gland by the action of a proteolytic enzyme. It is deiodinated in peripheral tissues to form triiodothyronine (T3), which has a greater biological activity.

Thyroxine acts as a catalyst in the body and influences a great variety of effects, including metabolic rate (oxygen consumption); growth and development; metabolism of carbohydrates, fats, proteins, electrolytes, and water; vitamin requirements; reproduction; and resistance to infection. Pharmaceutical preparations of thyroxine, extracted from animals or made synthetically, are called levothyroxine.free thyroxine the fraction of thyroxine in the serum that is not bound to a binding protein.