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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.
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:
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 ...
Decreased thyroxine-binding globulin occurs following bypass surgery, and in chronic illness, a less effective form of TBG with lower affinity for thyroxine is synthesized. Reduced quantities of bound thyroid result, leading to decreased total thyroid measurements.
Thyroxine-binding globulin (TBG) is a glycoprotein that has a higher affinity for T 4 than for T 3. Transthyretin is also a glycoprotein, but only carries T 4, with hardly any affinity at all for T 3. Finally, both hormones bind with a low affinity to serum albumin, but, due to the large availability of albumin, it has a high capacity.
"You can have a normal total testosterone level, but still suffer from symptoms of low T if your free testosterone is low," says Staheli. In fact, some research suggests that free T levels are a ...
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]
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]