Calcitonin primarily acts to:

• A. Increase osteoclast activity
• B. Decrease osteoclast activity
• C. Increase blood calcium levels
• D. Stimulate kidney function

Answer: (B)

Calcitonin is a hormone produced by the thyroid gland that plays a significant role in calcium homeostasis in the body. Its primary function is to decrease blood calcium levels. It achieves this by inhibiting the activity of osteoclasts, which are the cells responsible for breaking down bone and releasing calcium into the bloodstream. By reducing osteoclast activity, calcitonin effectively lowers the amount of calcium released from the bones, leading to a decrease in blood calcium levels.

In addition to its effects on osteoclasts, calcitonin also encourages the kidneys to excrete calcium, but its main action is on bone metabolism. Therefore, the correct function of calcitonin, particularly in relation to the question, is its role in decreasing osteoclast activity, which directly contributes to the regulation of calcium levels in the body. This action is crucial in maintaining bone density and preventing conditions such as osteoporosis.