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Thyroid hormones (T 4 and T 3) are produced by the follicular cells of the thyroid gland and are regulated by TSH made by the thyrotropes of the anterior pituitary gland. The effects of T 4 in vivo are mediated via T 3 (T 4 is converted to T 3 in target tissues). T 3 is three to five times more active than T 4.
When stimulated by thyroid stimulating hormone (TSH), these secrete the thyroid hormones T 3 and T 4. They do this by transporting and metabolising the thyroglobulin contained in the colloid. [ 4 ] Follicular cells vary in shape from flat to cuboid to columnar, depending on how active they are.
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 itself.
The pituitary gland secretes thyrotropin (TSH; Thyroid Stimulating Hormone) that stimulates the thyroid to secrete thyroxine (T4) and, to a lesser degree, triiodothyronine (T3). The major portion of T3, however, is produced in peripheral organs, e.g. liver, adipose tissue, glia and skeletal muscle by deiodination from
Thyroid hormone synthesis. [2]Follicular cells take up iodide and amino acids from the blood circulation on the basolateral side, synthesize thyroglobulin and thyroperoxidase from amino acids and secrete these into the thyroid follicles together with iodide.
T 3 is the more metabolically active hormone produced from T 4.T 4 is deiodinated by three deiodinase enzymes to produce the more-active triiodothyronine: . Type I present in liver, kidney, thyroid, and (to a lesser extent) pituitary; it accounts for 80% of the deiodination of T 4.
Thyroid stimulating hormone (TSH) is produced by the pituitary gland, another hormone-producing organ in the head. This in turn causes the thyroid to produce T3 and T4, which play a role in the ...
Parafollicular cells are also known to secrete in smaller quantities several neuroendocrine peptides such as serotonin, somatostatin or CGRP. [8] [9] [10] They may also have a role in regulating thyroid hormones production locally, as they express thyrotropin-releasing hormone. [11] [12]