triiodothyronine

triiodothyronine /tri·io·do·thy·ro·nine/ (tri″i-o″do-thi´ro-nēn) one of the thyroid hormones, an organic iodine-containing compound liberated from thyroglobulin by hydrolysis. It has several times the biological activity of thyroxine. Symbol T3.

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tri·i·o·do·thy·ro·nine (tr-d-thr-nn, -nn)

n.

See 3,5,3S-triiodothyronine.

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triiodothyronine (T₃)

[trī′ī·ō′dōthī′rənēn]

a hormone that helps regulate growth and development, helps control metabolism and body temperature, and, by a negative-feedback system, acts to inhibit the secretion of thyrotropin by the pituitary gland. Triiodothyronine is produced mainly from the deiodination of thyroxine in the peripheral tissues but is also synthesized by and stored in the thyroid gland as an amino acid residue of the protein thyroglobulin. Triiodothyronine circulates in the plasma, where it is bound mainly to thyroxine-binding globulin and thyroxine-binding prealbumin, proteins that protect the hormone from metabolism and excretion during its half-life of 2 days or less before it is degraded in the liver. The hormone is the most active thyroid hormone and affects all body processes, including gene expressions. It is a component of various drugs, such as liotrix and liothyronine sodium, used in the treatment of hypothyroidism and simple goiter. Normal adult blood levels are 110 to 230 ng/dL. See also thyroid hormone.

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triiodothyronine [tri″i-o″do-thi´ro-nēn]

a thyroid hormone that contains iodine and is liberated from thyroglobulin by hydrolysis. It has several times the biologic activity of thyroxine.

free triiodothyronine the fraction of triiodothyronine in the serum that is not bound to a binding protein.

triiodothyronine resin uptake test a thyroid function test, measuring how many sites on thyroxine-binding globulin (TBG) are occupied by endogenous triiodothyronine (T₃) and how many sites remain available. An excess of radioactive exogenous triiodothyronine is added to the sample, followed by the addition of a resin that also binds T₃. A portion of the radioactive T₃ binds to sites on TBG not already occupied by endogenous thyroid hormones, and the remainder binds to the resin. The amount of labeled hormones bound to the resin (the triiodothyronine resin uptake) can be subtracted from the total that was added and the remainder is the amount that bound to the unoccupied binding sites on the thyroxine-binding globulin.

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triiodothyronine (trī´īō´dōthī´r-nēn´),

n a hormone that helps regulate growth and development, control metabolism and body temperature, and by a negative-feedback system, inhibit the secretion of thyrotropin by the pituitary gland.

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triiodothyronine

T₃, 3,5,3´,-´triiodothyronine A hormone formed by removing an iodine ion from the chemical parent thyroxine–T₄, which occurs in the liver and kidney; in general, the T₃ and T₄ serum levels rise and fall together with certain exceptions–eg, T₃ thyrotoxicosis, in which T₄ and free T₄ values are in the normal range; most T₃ is not bound to a carrier protein in the circulation ↑ in Hyperthyroidism–T₃ thyrotoxicosis, pregnancy, and therapy with clofibrate,–oral contraceptives–progestins, estrogens, methadone, perphenazine ↓ in Euthyroid sick syndrome, ↑ free fatty acids, hypothyroidism, malnutrition, or therapy with corticosteroids, ethionamide, heparin, iodides, lithium, methimazole, phenylbutazone, phenytoin, propranolol, propylthiouracil, reserpine, salicylates, sulfonamides, testosterone, tolbutamide; Pts with T₃ thyrotoxicosis are clinically heterogeneous and lack distinctive signs and Sx, comprising 4% of hyperthyroid Pts due to Graves' disease, toxic nodular goiter and thyroid adenomas and a higher percentage of hyperthyroidism in regions with lower levels of iodine Ref value 0.92-2.46 nmol/L; 60-160 ng/dL. See Thyroxine–T_{4, Thyroxine-binding globulin, TSH.}