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Thyroid's secretory capacity (G T, also referred to as thyroid's incretory capacity, maximum thyroid hormone output, T4 output or, if calculated from serum levels of thyrotropin and thyroxine, as SPINA-GT [a]) is the maximum stimulated amount of thyroxine that the thyroid can produce in a given time-unit (e.g. one second).
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.
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
When circulating in the body, T3 and T4 are bound to transport proteins. Only a small fraction of the circulating thyroid hormones are unbound or free, and thus biologically active. T3 and T4 levels can thus be measured as free T3 and T4, or total T3 and T4, which takes into consideration the free hormones in addition to the protein-bound hormones.
This process, known as organification, results in the formation of monoiodotyrosine (MIT) and diiodotyrosine (DIT) residues within the Tg molecule. [ 6 ] [ 4 ] The final step in thyroxine synthesis involves the free radical mediated coupling of two DIT residues, catalyzed by TPO, to form T 4 while still attached to the Tg backbone.
The TRH test involves administration of a small amount of TRH intravenously, [1] following which levels of TSH will be measured at several subsequent time points using samples of blood taken from a peripheral vein. [citation needed] The test is used in the differential diagnosis of secondary and tertiary hypothyroidism.
Iodothyronine deiodinases (EC 1.21.99.4 and EC 1.21.99.3) are a subfamily of deiodinase enzymes important in the activation and deactivation of thyroid hormones. Thyroxine (T 4), the precursor of 3,5,3'-triiodothyronine (T 3) is transformed into T 3 by deiodinase activity.