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Steroid and Thyroid Hormones - The Medical …
Thyroid Stimulating Hormone (TSH): A pituitary hormone that triggers release of (T4) and (T3) from the thyroid gland, both of which play key roles in determining the body's metabolic rate and temperature, rate of protein synthesis and sensitivity to a class of substances called the best known example being the neurotransmitter/hormone or . TSH is inhibited by somatostatin.
Within the thyroid gland, epithelial cells synthesize thyroid hormones and are arranged as thyroid follicles. Between the thyroid follicles are parafollicular (alternatively, C cells), which secrete the hormone calcitonin. Tg is secreted by the thyroid cell into the follicular lumen by regulated (nonconstitutive), merocrine secretion (;). Upon stimulation by thyroid-stimulating hormone (TSH), Tg is reabsorbed by endocytosis/pinocytosis or phagocytosis (rodents only) to form endocytic/pinocytic vesicles or phagosomes, respectively (;). Thyroid hormone synthesis is regulated by the hypothalamic-pituitary-thyroid axis (HPT) negative feedback system, which involves interactions between the thyroid gland and circulating hormones. In the HPT, thyroid hormone synthesis is induced by TSH, which is secreted from anterior pituitary endocrine cells called thyrotropes. Increased thyroid hormone levels suppress further TSH secretion by acting on the hypothalamus, subsequently inhibiting the release of TSH-releasing hormone (TRH). TSH binding to the TSHR at the basal membrane of the thyroid follicle stimulates Tg expression via an increase in the intracellular level of cyclic adenosine monophosphate (cAMP) as well as the stimulation of thyroid peroxidase (TPO) and the sodium/iodide symporter (NIS) (). After synthesis, Tg is transported to the apical membrane whereby it is released into the follicular lumen. At the apical membrane TPO and H2O2 catalyze the iodination of tyrosine residues on Tg to produce mono- (MIT) and di-iodotyrosines (DIT). Subsequent coupling of MIT and DIT catalyzes the production of T3 and T4. The iodinated Tg is pinocytosed through the apical membrane and into lysosomes where it undergoes proteolysis to release T3 and T4 through the basal membrane into the blood for action at peripheral target tissues whereby they control metabolism ().
Glossary | Linus Pauling Institute | Oregon State University
Tg synthesis is restricted to the thyroid gland. Tg is secreted and stored in the colloid inside of the thyroid follicles. Newly secreted Tg remains near the apical membrane of thyroid cells, where it undergoes hormone formation and is internalized and/or degraded rapidly. Tg in the center of the follicle is stored in the form of aggregates to function as an iodine and hormone reservoir.
The thyroid hormones act on nearly every cell in the body
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