What hormone primarily regulates blood calcium levels?

Parathyroid hormone, or PTH, is the primary hormone that regulates blood calcium levels.

Four small parathyroid glands behind the thyroid monitor calcium in the blood. When calcium falls, they release PTH. The hormone raises blood calcium by acting on bone, kidneys, and the intestine. In bone, it promotes a signaling pathway that increases resorption, releasing stored calcium. In the kidneys, it reduces calcium loss in urine and encourages activation of vitamin D.

Active vitamin D, called calcitriol, increases calcium absorption from the intestine. This coordinated response restores calcium homeostasis, which is essential for muscle contraction, nerve signaling, blood clotting, and bone maintenance. PTH also lowers blood phosphate partly by increasing phosphate excretion through the kidneys.

Calcitonin is the common distractor because it can lower blood calcium, but it is not the main day-to-day regulator in humans. The thyroid and parathyroid glands are separate organs despite their names, and PTH is produced by the parathyroids, not the thyroid.