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free thyroxine |
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free thyroxine, the amount of the unbound, active thyroid hormone thyroxine circulating in the blood, measured by specific laboratory procedures. See also free thyroxine index. thyroxine, thyroxin a hormone of the thyroid gland that contains iodine and is a derivative of the amino acid tyrosine. The chemical name for thyroxine is tetraiodothyronine (symbol, T4); it is formed and stored in the thyroid follicles as thyroglobulin, the storage form. Thyroxine is released from the gland by the action of a proteolytic enzyme. T4 is deiodinated in peripheral tissues to form tri-iodothyronine (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. Thyroxine can be extracted from animals or made synthetically; it is used in the treatment of hypothyroidism and some types of goiter. free thyroxine the metabolically active fraction of thyroxine; abbreviated FT4, Tf. T4 = Tf + TBG. The amount is very small and difficult to estimate so that the amount present in serum is not used as a more accurate indicator of thyroxine status than T4. thyroxine-binding globulin (TBG) most (99.95%) of the thyroxine in plasma is bound to globulin and a small amount bound to prealbumin. thyroxine-binding prealbumin bound to a small fraction of circulating T4. This is the only known function of prealbumins. L-thyroxine see levothyroxine. How to thank TFD for its existence? Tell a friend about us, add a link to this page, add the site to iGoogle, or visit webmaster's page for free fun content. |
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| ? Mentioned in | ? References in periodicals archive | ||
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| One result of thyroid suppression therapy with levothyroxine is subclinical thyrotoxicosis (subclinical hyperthyroidism), defined as the presence of a low TSH level with a normal free thyroxine (F[T. Propylthiouracil-induced hypothyroidism in coho salmon, Oncorhynchus kisutch: effects on plasma total thyroxine, total triiodothyronine, free thyroxine and growth hormone. The serum free thyroxine index increased significantly, but there was no change in serum |
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