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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 function tests (TFTs) is a collective term for blood tests used to check the function of the thyroid. [1] TFTs may be requested if a patient is thought to suffer from hyperthyroidism (overactive thyroid) or hypothyroidism (underactive thyroid), or to monitor the effectiveness of either thyroid-suppression or hormone replacement therapy.
Thyroxine-binding globulin (TBG) is a globulin protein encoded by the SERPINA7 gene in humans. TBG binds thyroid hormones in circulation.It is one of three transport proteins (along with transthyretin and serum albumin) responsible for carrying the thyroid hormones thyroxine (T 4) and triiodothyronine (T 3) in the bloodstream.
In addition to testing the TSH levels, many doctors test for T 3, Free T 3, T 4, and/or Free T 4 for more detailed results. Free T 4 is unbound to any protein in the blood. Adult limits for these hormones are: TSH (units): 0.45 – 4.50 uIU/mL; T 4 Free/Direct (nanograms): 0.82 – 1.77 ng/dl; and T 3 (nanograms): 71 – 180 ng/dl.
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.
Fasting in healthy, euthyroid people causes reduced T3 and elevated rT3, although TSH is usually unchanged. [3] [5] [10] Even moderate weight loss can lower T3. [2] This may be primarily via reduced levels of leptin (the satisfaction hormone). Low leptin levels can downregulate hypothalamic TRH neurons and cause a reduction in TSH.
Tiratricol is an analogue of a naturally circulating metabolite of the active thyroid hormone T3. [1] MCT8 is a specific thyroid hormone transporter. [ 1 ] While T3 and T4 thyroid hormones rely on MCT8 to enter several tissues such as the brain, tiratricol can enter cells independently of MCT8. [ 1 ]
Reverse triiodothyronine, also known as rT 3, is an isomer of triiodothyronine (T 3).. Reverse T 3 is the third-most common iodothyronine the thyroid gland releases into the bloodstream, at 0.9%; tetraiodothyronine (levothyroxine, T 4) constitutes 90% and T 3 is 9%.