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Calcitonin is a 32 amino acid peptide hormone secreted by parafollicular cells (also known as C cells) of the thyroid (or endostyle) in humans and other chordates [5] in the ultimopharyngeal body. [6] It acts to reduce blood calcium (Ca 2+), opposing the effects of parathyroid hormone (PTH). [7]
At the same time, the parathyroid glands reduce the secretion of parathyroid hormone (PTH), also a polypeptide hormone, into the blood. The resulting high levels of calcitonin in the blood stimulate osteoblasts in bone to remove calcium from blood plasma and deposit it as bone. The reduced levels of PTH inhibit removal of calcium from the skeleton.
Calcitonin lowers blood levels of calcium by inhibiting the resorption of bone by osteoclasts, and its secretion is increased proportionally with the concentration of calcium. [ 7 ] Parafollicular cells are also known to secrete in smaller quantities several neuroendocrine peptides such as serotonin , somatostatin or CGRP .
Teriparatide, sold under the brand name Forteo, is a form of parathyroid hormone (PTH) consisting of the first 34 amino acids, which is the bioactive portion of the hormone. [41] It is an effective anabolic (promoting bone formation) agent [ 42 ] used in the treatment of some forms of osteoporosis .
Calcitonin-producing parafollicular cells are also found along the basement membrane of the thyroid follicle, interspersed between follicular cells; and in spaces between the spherical follicles. Parafollicular cells can be distinguished from follicular cells based on their lighter staining cytoplasm using H&E .
The effects of calcitonin are opposite those of the parathyroid hormone (PTH) produced in the parathyroid glands. However, calcitonin seems far less essential than PTH, since calcium metabolism remains clinically normal after removal of the thyroid ( thyroidectomy ), but not the parathyroid glands .
Parathyroid hormone (PTH) is a polypeptidic hormone primarily involved in calcium metabolism. The parathyroid hormone-related protein (PTH-rP) is a related protein with predominantly paracrine function and possibly an endocrine role in lactation, as PTHrP has been found to be secreted by mammary glands into the circulation and increase bone ...
PTH acts on bone, the kidneys, and the GI tract to increase calcium reabsorption and phosphate excretion. In addition, PTH stimulates the conversion of Vitamin D to its most active variant, 1,25-dihydroxyvitamin D 3, which further stimulates calcium absorption in the GI tract. [3] Thymus Gland; Adrenal glands. Adrenal cortex; Adrenal medulla ...