Parathyroid hormone is secreted from four parathyroid glands, which are small glands in the neck, located behind the thyroid gland. Parathyroid hormone regulates calcium levels in the blood, largely by increasing the levels when they are too low. It does this through its actions on the kidneys, bones and intestine:
Bones – Parathyroid hormone acts on a bone to increase its blood calcium levels by stimulating osteoclasts to break down bone and release calcium into the bloodstream.
Kidneys – parathyroid hormone acts on the kidneys to increase blood calcium levels by promoting calcium reabsorption in the nephrons.
Intestine – Parathyroid hormone acts on the gastrointestinal tract to increase blood calcium by increasing the activity of the enzyme in the intestines that activates vitamin D.
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