Which of the following hormones maintains the level of calcium in blood?

Concept: Blood calcium concentration is kept within a narrow range by a dedicated endocrine axis. Parathyroid hormone (parathormone, PTH), secreted by the parathyroid glands, is the principal hormone that raises and maintains the level of calcium in the blood when it falls.

Application: When blood calcium drops, the parathyroid glands release parathormone, which restores the level through three coordinated actions:

1. It stimulates osteoclasts to resorb bone matrix, releasing stored calcium into the bloodstream.

2. It increases calcium reabsorption in the kidney's distal tubules while promoting phosphate excretion.

3. It stimulates the kidney to activate vitamin D into calcitriol, which enhances calcium absorption from the intestine.

Contrast with the other options:

• Thyroxin, secreted by the thyroid gland, governs the body’s basal metabolic rate and overall growth/development; it does not directly regulate blood calcium level.

• Prolactin, secreted by the anterior pituitary, stimulates milk production during lactation; it plays no role in calcium homeostasis.

• Somatotropin (growth hormone), secreted by the anterior pituitary, promotes overall body growth and protein synthesis; it does not target blood calcium regulation.

Cross-check: The thyroid gland’s C-cells secrete a separate hormone, calcitonin, which lowers blood calcium when it rises too high — the opposite action of parathormone. This antagonist pair (parathormone raises, calcitonin lowers) confirms that calcium regulation is governed by a dedicated PTH-calcitonin axis, distinct from thyroxine, prolactin, or growth hormone, confirming parathormone as the hormone that maintains the level of calcium in blood.