Parathyroid hormone is the most important endocrine regulator of calcium and phosphorus concentration in extracellular fluid. This hormone is secreted from cells of the parathyroid glands and finds its major target cells in bone and kidney. Another hormone, parathyroid hormone-related protein, binds to the same receptor as parathyroid hormone and has major effects on development.
Like most other protein hormones, parathyroid hormone is synthesized as a preprohormone. After intracellular processing, the mature hormone is packaged within the Golgi into secretory vesicles, the secreted into blood by exocytosis. Parathyroid hormone is secreted as a linear protein of 84 amino acids.
Physiologic Effects of Parathyroid Hormone
Writing a job description for parathyroid hormone is straightforward: if calcium ion concentrations in extracellular fluid fall below normal, bring them back within the normal range. In conjunction with increasing calcium concentration, the concentration of phosphate ion in blood is reduced. Parathyroid hormone accomplishes its job by stimulating at least three processes:
Addition information on how parathyroid hormone and vitamin D control calcium balance can be found in the section Endocrine Control of Calcium Homeostasis.
Control of Parathyroid Hormone Secretion
Parathyroid hormone is released in response to low extracellular concentrations of free calcium. Changes in blood phosphate concentration can be associated with changes in parathyroid hormone secretion, but this appears to be an indirect effect and phosphate per se is not a significant regulator of this hormone.
When calcium concentrations fall below the normal range, there is a steep increase in secretion of parathyroid hormone. Low levels of the hormone are secreted even when blood calcium levels are high. The figure to the right depicts parathyroid hormone release from cells cultured in vitro in differing concentrations of calcium.
The parathyroid cell monitors extracellular free calcium concentration via an integral membrane protein that functions as a calcium-sensing receptor.
Both increased and decreased secretion of parathyroid hormone are recognized as causes of serious disease in man and animals.
Excessive secretion of parathyroid hormone is seen in two forms:
There is no doubt that chronic secretion or continuous infusion of parathyroid hormone leads to decalcification of bone and loss of bone mass. However, in certain situations, treatment with parathyroid hormone can actually stimulate an increase in bone mass and bone strength. This seemingly paradoxical effect occurs when the hormone is administered in pulses (e.g. by once daily injection), and such treatment appears to be an effective therapy for diseases such as osteoporosis.
Inadequate production of parathyroid hormone - hypoparathyroidism - typically results in decreased concentrations of calcium and increased concentrations of phosphorus in blood. Common causes of this disorder include surgical removal of the parathyroid glands and disease processes that lead to destruction of parathyroid glands. The resulting hypocalcemia often leads to tetany and convulsions, and can be acutely life-threatening. Treatment focuses on restoring normal blood calcium concentrations by calcium infusions, oral calcium supplements and vitamin D therapy.
Advanced and Supplemental Topics
|Index of: Thyroid and Parathyroid Glands|
|Calcitonin||Introduction and Index|
Last updated on September 28, 2003
|Author: R. Bowen|
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