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At the cellular level, T 3 is the body's more active and potent thyroid hormone. [2] T 3 helps deliver oxygen and energy to all of the body's cells, its effects on target tissues being roughly four times more potent than those of T 4. [2] Of the thyroid hormone that is produced, just about 20% is T 3, whereas 80% is produced as T 4.
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]
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]
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.
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 .
The follicular cells subsequently take up iodinated thyroglobulin from the follicles by endocytosis, extract thyroid hormones from it with the help of proteases and subsequently release thyroid hormones into the blood. These thyroid hormones are transported throughout the body where they control metabolism (which is the conversion of oxygen and ...
Parafollicular cells, also called C cells, are neuroendocrine cells in the thyroid. They are called C cells because the primary function of these cells is to secrete calcitonin. [1] They are located adjacent to the thyroid follicles and reside in the connective tissue. These cells are large and have a pale stain compared with the follicular cells.
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 ]