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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.
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).
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 Thyrotroph Thyroid Hormone Sensitivity Index (abbreviated TTSI, also referred to as Thyrotroph T4 Resistance Index or TT4RI) is a calculated structure parameter of thyroid homeostasis. It was originally developed to deliver a method for fast screening for resistance to thyroid hormone.
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
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 hormones act on nearly every cell in the body. They act to increase the basal metabolic rate, affect protein synthesis, help regulate long bone growth (synergy with growth hormone) and neural maturation, and increase the body's sensitivity to catecholamines (such as adrenaline) by permissiveness. [12]
Levothyroxine should be used to keep TSH levels within the normal range for that trimester. The first-trimester normal range is below 2.5 mIU/L and the second and third trimesters normal range is below 3.0 mIU/L. [16] [44] Treatment should be guided by total (rather than free) thyroxine or by the free T 4 index.