Which thyroid hormone lowers blood calcium levels by stimulating osteoblasts to build bone?

• A. Calcitonin
• B. Parathyroid hormone
• C. Thyroxine
• D. Calcitriol

Answer: (A)

Calcium balance is controlled by hormones that shift calcium between bone and blood during bone remodeling. Calcitonin lowers blood calcium levels by acting on bone to prevent calcium release: it inhibits osteoclast activity, which reduces bone resorption, and supports the deposition of calcium into bone. This causes a net move of calcium from blood into bone, lowering serum calcium. Calcitonin is produced by the C cells of the thyroid, which is why it's described as a thyroid hormone in this context.

The other options don’t fit this mechanism as neatly. Parathyroid hormone raises blood calcium by stimulating osteoclasts to break down bone and by increasing intestinal and renal calcium absorption. Calcitriol (vitamin D) increases calcium absorption from the gut and can enhance bone turnover, which tends to raise or regulate calcium levels rather than lower them. Thyroxine mainly increases metabolic activity and can accelerate bone turnover, potentially contributing to bone loss if in excess, rather than specifically lowering calcium through stimulating bone formation.