Which hormone inhibits osteoclast activity?

• A. Calcitonin
• B. Parathyroid hormone
• C. Growth hormone
• D. Testosterone

Answer: (A)

Bone remodeling is controlled by hormones that tune the activity of osteoclasts and osteoblasts. Calcitonin, produced by thyroid C cells, is released when blood calcium is high and acts to slow bone resorption. It binds receptors on osteoclasts, reducing their activity and formation, which lowers the amount of calcium released from bone and helps bring serum calcium down. This direct inhibition of osteoclasts is why calcitonin most specifically reduces bone resorption.

In contrast, parathyroid hormone raises blood calcium largely by stimulating osteoclast-mediated bone resorption through signals from osteoblasts, so it promotes osteoclast activity rather than inhibiting it. Growth hormone tends to have anabolic effects on bone, predominantly promoting osteoblast activity and bone formation rather than directly suppressing osteoclasts. Testosterone supports bone density primarily through formation and overall balance of remodeling, not by directly inhibiting osteoclasts.