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A blood test for free T3 triiodothyronine what is it?


Triiodothyronine ( T 3 - triiodothyronine; see also Thyroxine and Thyroid- stimulating hormone ). T 3 is a thyroid hormone circulating in the blood. Only 20% of triiodothyronine (T 3 ) enters the blood in a finished form from the thyroid gland. The predominant quantitative thyroxine (T 4 ) is converted in the liver and in peripheral tissues to the more active T 3 . Both hormones T 3 and T 4 are found in the blood for the most part associated with protein (TBG - thyroxine binding globuline - thyroxine binding globulin). Normal serum triiodothyronine is 70–190 ng / dL. It rises in hyperthyroidism and decreases in hypothyroidism - this is the main clinical significance of the test. During pregnancy, it is always elevated, not reflecting the functional state of the thyroid gland (unlike T 4 ).       

Ion exchange resin capture triiodothyronine (RT 3 U - resin triiodothyro - nine uptake) . The indicator of this test is normal = 35–38%. This test is based on the fact that a preparation of radioactive iodine with triiodothyronine (T 3 ) placed in the patient's serum (outside the body) is partially bound by the same globulin (thyroxine binding globulin TBG), to which T 3 and T 4 are associated (see above). After that, an ion-exchange resin is added, which attaches the remaining part of the preparation T 3 that has not bound to globulin . The amount of triiodothyronine (T 3 ) removed by the resin is used to judge the binding capacity of TBG globulin. The more T 3 was removed with the resin, the less it was bound to TBG. If we assume that this is because the binding capacity of globulin was occupied by a larger than normal amount of hormones T 3 and T 4 in the blood , then an increase in the test result indicates hyperthyroidism, and a decrease — of hypothyroidism. This conclusion is often correct. But the binding capacity of TBG globulin can increase or decrease due to changes in the amount or properties of TBG itself. Therefore, the indicator of this test is not specific. In addition to diseases of the thyroid gland, it grows with renal failure, nephrosis, protein deficiency, and decreases, for example, with hepatitis, during pregnancy, and with some tumors.  

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