What is the function of calcitonin in relation to calcium levels in the body?

• A. It increases calcium levels in the blood
• B. It tones down calcium levels in the blood
• C. It has no effect on calcium levels
• D. It regulates phosphorus levels

Answer: (B)

Calcitonin is a hormone produced by the parafollicular cells (or C cells) of the thyroid gland that plays a crucial role in regulating calcium levels in the body. When calcium levels in the blood rise, calcitonin is secreted to help lower these levels. It does this primarily by inhibiting osteoclast activity, which are the cells responsible for breaking down bone and releasing calcium into the bloodstream. Additionally, calcitonin can decrease calcium reabsorption in the kidneys, promoting its excretion in urine.

Thus, the correct answer reflects the hormone's function of toning down calcium levels by counteracting the effects of parathyroid hormone (which raises calcium levels) and maintaining calcium homeostasis. This hormonal balance is key to many physiological processes, including bone metabolism and muscle function.