The secretion of both calcitonin and parathyroid hormone is determined by the level of calcium in the blood. When levels of calcium in the blood increase, calcitonin is secreted in higher quantities. When levels of calcium in the blood decrease, this causes the amount of calcitonin secreted to decrease too.
The secretion of calcitonin is also inhibited by the hormone somatostatin , which can also be released by the C-cells in the thyroid gland. There does not seem to be any direct deleterious effect on the body as a result of having too much calcitonin. Medullary thyroid cancer is a rare type of cancer that arises from the C-cells in the thyroid gland that secrete calcitonin. It is sometimes associated with multiple endocrine neoplasia type 2a and multiple endocrine neoplasia type 2b.
Patients with medullary thyroid cancer have high calcitonin levels in their bloodstream. However, it is important to note that these high calcitonin levels are a consequence of this condition, not a direct causal factor. There does not seem to be any clinical effect on the body as a result of having too little calcitonin.
Precise maintenance of the physiologic levels of both extracellular and intracellular ionized calcium is essential to life. Calcium and phosphate homeostasis is complex, yet three important hormones are responsible for modulating most of the extracellular control of these minerals. Parathyroid hormone acts directly on bone and kidney and indirectly on the intestine to maintain or restore the serum calcium level.
The signal for increased PTH synthesis and secretion is a decrease in the serum ionized calcium concentration and a decrease in serum levels of 1,25 OH 2-D. The remainder of body phosphate is present in a variety of inorganic and organic compounds distributed within both intracellular and extracellular compartments. Normal blood concentrations of phosphate are very similar to calcium.
Maintaining constant concentrations of calcium in blood requires frequent adjustments, which can be described as fluxes of calcium between blood and other body compartments. Three organs participate in supplying calcium to blood and removing it from blood when necessary:. Maintaining normal blood calcium and phosphorus concentrations is managed through the concerted action of three hormones that control fluxes of calcium in and out of blood and extracellular fluid:.
Parathyroid hormone serves to increase blood concentrations of calcium. Mechanistically, parathyroid hormone preserves blood calcium by several major effects:. Vitamin D acts also to increase blood concentrations of calcium. This results in the removal of calcium from the bone to correct blood calcium levels. Learning Objectives Describe the factors involved in calcium homeostasis. Key Points Calcium homeostasis regulates calcium flow to and from the bones. Inadequate calcium levels can result in osteoporosis.
Vitamin D is converted to calcidiol also called calcifediol in the liver, which is then converted to calcitriol in the kidneys, the biologically active form of vitamin D. Calcitriol regulates the levels of calcium and phosphorus in the blood and helps maintain a healthy skeletal system.
Bone resorption by osteoclasts releases calcium into the bloodstream, which helps regulate calcium homeostasis. Key Terms calcitriol : The active metabolite—1,dihydroxycholecalciferol—that is involved in the absorption of calcium.
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