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14.11: Parathormone

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    Sources and Control of Secretion

    Parathormone is produced by the parathyroid gland, which consists of several clusters of cells on the back of the thyroid gland (Figure 14.1). The secretion of parathormone is controlled by a negative feedback system similar to the one regulating calcitonin secretion. However, unlike calcitonin, parathormone secretion is stimulated by low blood calcium and inhibited by high blood calcium.


    Parathormone raises blood calcium levels by causing several alterations in target cell activities. It stimulates the removal of calcium from bone matrix; reduces the release of calcium by the kidneys into the urine; stimulates directly the absorption of calcium by the small intestine; and stimulates the activation of vitamin D by the kidneys. The increase in active vitamin D aids the absorption of calcium by the small intestine.

    Note that parathormone directly antagonizes the effects of calcitonin. Maintenance of blood calcium homeostasis depends on providing a proper balance between these two hormones, proper functioning of their target structures, and adequate supplies of active vitamin D and dietary calcium. Besides building and maintaining bone matrix, homeostasis of blood calcium is important for cell and muscle movements, nervous impulse transmission, blood clotting, and regulation of cell activities.

    Age Changes

    During aging, the parathyroid gland retains the ability to increase or decrease the secretion of parathormone in response to changes in blood calcium levels. Blood levels of active parathormone either do not change or increase slightly. Paradoxically, blood calcium levels decline, though they remain satisfactory. This decline may result from decreases in dietary calcium intake and vitamin D levels.

    Individuals with increased parathormone levels may be at greater risk for both rapid loss of bone matrix and osteoporosis. In addition, as noted for calcitonin, the drop in estrogen levels in postmenopausal women may have adverse effects on the ability of the parathyroid gland and parathormone to regulate blood calcium and to help maintain bone matrix.

    This page titled 14.11: Parathormone is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Augustine G. DiGiovanna via source content that was edited to the style and standards of the LibreTexts platform.

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