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Jostel's TSH index (TSHI or JTI), also referred to as Jostel's thyrotropin index or Thyroid Function index (TFI), is a method for estimating the thyrotropic (i.e. thyroid stimulating) function of the anterior pituitary lobe in a quantitative way. [1] [2] The equation has been derived from the logarithmic standard model of thyroid homeostasis.
It is the primary form of thyroid hormone found in the blood and acts as a prohormone of the more active thyroid hormone, triiodothyronine (T 3). [1] Thyroxine and its active metabolites are essential for regulating metabolic rate , supporting heart and muscle function , promoting brain development , and maintaining bone health .
TSH (with a half-life of about an hour) stimulates the thyroid gland to secrete the hormone thyroxine (T 4), which has only a slight effect on metabolism. T 4 is converted to triiodothyronine (T 3), which is the active hormone that stimulates metabolism. About 80% of this conversion is in the liver and other organs, and 20% in the thyroid ...
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]
Three primary deiodinases are responsible for thyroid hormone conversion and breakdown. Type 1 (D1) deiodinates T4 to the biologically active T3, as well as the hormonally inactive and possibly inhibitory rT3. [3] [5] Type 2 (D2) converts T4 into T3, and breaks down rT3. D3 produces rT3 from T4, and breaks down T3.
3,5-T 2 is an active thyroid hormone. It stimulates the TR-beta receptor for thyroid hormones and thus increases energy expenditure. [1] [2] It has agonistic (thyromimetic) effects at myocardial tissue and pituitary, which results in 3,5-T 2 suppressing TSH release.
3,3'-Diiodothyronine, also known as 3,3'-T 2, is a metabolite of thyroid hormone. It is formed from the breakdown of triiodothyronine . Levels can be affected in certain disease states.
[1] Thyroid hormones can have a direct inhibitory effect on thyrotropic cells, though the exact mechanism is unknown. At elevated levels of thyroxine, the rate of secretion of TSH decreases to near zero, as the body tries to maintain a relatively constant level of thyroid hormone in circulation. [1]