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Low-T3 syndrome and high-T3 syndrome: Consequences of step-up hypodeiodination, e.g. in critical illness as an example for type 1 allostasis, [20] or hyperdeiodination, as in type 2 allostasis, including posttraumatic stress disorder. [12] Resistance to thyroid hormone: Feedback loop interrupted on the level of pituitary thyroid hormone receptors.
Thyroxine has a half-life of approximately one week and hence maintains relatively stable blood levels. Its production and release are controlled through a complex feedback loop involving the hypothalamus, pituitary gland, and thyroid gland. This regulatory system ensures that optimal hormone levels are maintained. [4]
The thyroid system of the thyroid hormones T 3 and T 4 [1] Thyroid hormones are two hormones produced and released by the thyroid gland, triiodothyronine (T 3) and thyroxine (T 4). They are tyrosine-based hormones that are primarily responsible for regulation of metabolism. T 3 and T 4 are partially composed of iodine, derived from food. [2]
At 20 weeks, the fetus is able to implement feedback mechanisms for the production of thyroid hormones. During fetal development, T 4 is the major thyroid hormone being produced while triiodothyronine (T 3) and its inactive derivative, reverse T 3, are not detected until the third trimester. [2]
At 20 weeks, the fetus is able to implement feedback mechanisms for the production of thyroid hormones. During fetal development, T 4 is the major thyroid hormone being produced while triiodothyronine (T 3 ) and its inactive derivative, reverse T 3 , are not detected until the third trimester.
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 ]
Organification is a biochemical process that takes place in the thyroid gland. It is the incorporation of iodine into thyroglobulin for the production of thyroid hormone, a step done after the oxidation of iodide by the enzyme thyroid peroxidase (TPO) [1] Since iodine is an inorganic compound, and is being attached to thyroglobulin, a protein, the process is termed as "organification of iodine".
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 ...