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The thyroid system of the thyroid hormones T 3 and T 4 [1] Thyroid hormones are any hormones produced and released by the thyroid gland, namely 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]
Thyroid hormones are important for normal development. [28] They increase the growth rate of young people, [29] and cells of the developing brain are a major target for the thyroid hormones T 3 and T 4. Thyroid hormones play a particularly crucial role in brain maturation during fetal development and first few years of postnatal life [28]
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.
The TSH, in turn, stimulates the thyroid to produce thyroid hormone until levels in the blood return to normal. Thyroid hormone exerts negative feedback control over the hypothalamus as well as anterior pituitary, thus controlling the release of both TRH from hypothalamus and TSH from anterior pituitary gland. [2]
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 ]
A hormone receptor is a receptor molecule that binds to a specific hormone. Hormone receptors are a wide family of proteins made up of receptors for thyroid and steroid hormones, retinoids and Vitamin D, and a variety of other receptors for various ligands, such as fatty acids and prostaglandins. [1] Hormone receptors are of mainly two classes.
Hormones that use intracellular receptors include thyroid, aldosterone, and steroid hormones. [ 1 ] Examples are the class of nuclear receptors located in the cell nucleus and cytoplasm and the IP 3 receptor located on the endoplasmic reticulum .
The production of hormone by the thyroid gland is controlled by the hypothalamus and pituitary gland. The physiological activity of thyroid hormone is regulated by a system of enzymes that activate, inactivate or simply discard the prohormone T 4 and in turn functionally modify T 3 and rT 3. These enzymes operate under complex direction of ...