Hormone from the thyroid that lowers blood calcium.

• A. Parathyroid Hormone
• B. Calcitriol
• C. Calcitonin
• D. Thyroid Hormone

Answer: (C)

Calcium balance in the blood is controlled by hormones that either raise or lower calcium levels. Calcitonin, produced by the parafollicular cells of the thyroid, is released when blood calcium is high. It lowers calcium by two main actions: inhibiting osteoclasts in bone, which reduces bone resorption, and increasing calcium excretion by the kidneys. The net effect is a decrease in blood calcium. The other options don’t fit this pattern: parathyroid hormone raises blood calcium by stimulating bone resorption, increasing kidney reabsorption, and promoting activation of vitamin D to boost gut absorption; calcitriol (active vitamin D) mainly increases calcium absorption from the gut; thyroid hormone influences metabolic rate but does not directly regulate blood calcium. So the hormone described is calcitonin.