What is the primary effect of calcitonin in the body?

Calcitonin plays a significant role in calcium homeostasis by primarily lowering blood calcium levels. It achieves this effect through the inhibition of osteoclast activity, which are the cells responsible for bone resorption. When osteoclast activity is decreased, there is a reduced release of calcium from the bones into the bloodstream, leading to lowered serum calcium levels.

Calcitonin is secreted by the parafollicular cells (C cells) of the thyroid gland in response to elevated plasma calcium levels. In addition to its action on osteoclasts, it can also promote the reabsorption of calcium in the kidneys, further aiding in the reduction of blood calcium concentration.

In contrast, some choices suggest mechanisms that are not aligned with the known functions of calcitonin. For instance, increasing blood calcium levels is the opposite of what calcitonin does, and “having no effect” or “converting calcium into bone tissue” do not accurately describe its physiological role. Therefore, the correct answer reflects the fundamental action of calcitonin in regulating calcium homeostasis.