Which hormone is secreted by the parathyroid glands to regulate calcium levels?

• A. Calcitonin
• B. Parathyroid hormone
• C. Thyroid-stimulating hormone
• D. Melatonin

Answer: (B)

Calcium balance in the body is maintained by parathyroid hormone. When blood calcium is low, the parathyroid glands secrete this hormone, which acts to raise calcium back toward normal through three main effects: it stimulates bone to release calcium by promoting osteoclast activity, increases calcium reabsorption in the kidneys, and enhances production of active vitamin D (calcitriol) in the kidneys, which boosts calcium absorption from the intestine. This coordinated response elevates serum calcium and is regulated by a negative feedback loop—rising calcium levels suppress further PTH release.

The other options don’t fit this role: calcitonin, produced by the thyroid, lowers blood calcium; thyroid-stimulating hormone comes from the pituitary and governs thyroid hormone production; melatonin from the pineal gland regulates circadian rhythms.