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Thyroid hormone uptake (T uptake or T 3 uptake) is a measure of the unbound thyroxine binding globulins in the blood, that is, the TBG that is unsaturated with thyroid hormone. [2] Unsaturated TBG increases with decreased levels of thyroid hormones. It is not directly related to triiodothyronine, despite the name T 3 uptake. [2] Reference ranges:
Thyroid hormone binding ratio (THBR) is a thyroid function test that measures the "uptake" of T3 or T4 tracer by thyroid-binding globulin (TBG) in a given serum sample. This provides an indirect and reciprocal estimate of the available binding sites on TBG within the sample. The results are then reported as a ratio to normal serum.
Due to the very low concentration of T 4 and T 3 in the blood, TBG is rarely more than 25% saturated with its ligand. Unlike transthyretin and albumin, TBG has a single binding site for T 4 /T 3. TBG is synthesized primarily in the liver as a 54-kDa protein. In terms of genomics, TBG is a serpin; however, it has no inhibitory function like many ...
After exercise, a transient increase occurs in TSH, T4, and T3, but this is thought to be due to increased blood concentration as a result of dehydration. [2] The effects normalize after rest. After long-term heavy strain, levels of thyroid hormones decrease. [ 2 ]
The saturation of binding spots on thyronine-binding globulin (TBG) by endogenous T 3 can be estimated by the triiodothyronine resin uptake test. The test is performed by taking a blood sample, to which an excess of radioactive exogenous T 3 is added, followed by a resin that also binds T 3.
They can also be measured as total T 3 and total T 4, which depend on the amount that is bound to thyroxine-binding globulin (TBG). [74] A related parameter is the free thyroxine index, which is total T 4 multiplied by thyroid hormone uptake, which, in turn, is a measure of the unbound TBG. [75]
Most are bound to thyroxine-binding globulin (about 70%), transthyretin (10%), and albumin (15%). [30] Only the 0.03% of T 4 and 0.3% of T 3 traveling freely have hormonal activity. [31] In addition, up to 85% of the T 3 in blood is produced following conversion from T 4 by iodothyronine deiodinases in organs around the body. [24]
Thyroglobulin (Tg) is a 660 kDa, dimeric glycoprotein produced by the follicular cells of the thyroid and used entirely within the thyroid gland. Tg is secreted and accumulated at hundreds of grams per litre in the extracellular compartment of the thyroid follicles, accounting for approximately half of the protein content of the thyroid gland. [5]