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Parathyroid hormone, also known as parathormone or PTH, is a hormone produced by the parathyroid glands, which are four small glands located behind the thyroid gland in the neck. The primary role of PTH is to regulate calcium levels in the blood. It acts on the bones, kidneys, and intestines to maintain calcium homeostasis.

When blood calcium levels drop too low, the parathyroid glands release more PTH. This hormone then signals bone cells to release calcium from the bones into the bloodstream. In the kidneys, PTH promotes the reabsorption of calcium, preventing its excretion in urine. It also stimulates the kidneys to produce calcitriol, the active form of vitamin D, which then increases the absorption of calcium from the intestines.

Conversely, when blood calcium levels are too high, PTH secretion is suppressed, leading to a decrease in calcium release from bones and increased calcium excretion by the kidneys. This intricate feedback mechanism ensures that blood calcium levels remain within a narrow and essential range, which is critical for various bodily functions, including nerve transmission, muscle contraction, and blood clotting. Imbalances in PTH production can lead to hyperparathyroidism (excess PTH) or hypoparathyroidism (insufficient PTH), both of which can have significant health consequences.