If a thyroid tumor secreted an excessive amount of calcitonin, what would be expected in the body?

• A. An elevated level of osteoblast activity
• B. A decrease in bone density
• C. An increase in osteoclast activity
• D. A reduced calcium level in the blood

Answer: (A)

When a thyroid tumor secretes an excessive amount of calcitonin, the most likely consequence is a reduction of calcium in the blood. Calcitonin is a hormone produced by the parafollicular cells (C cells) of the thyroid gland, and its primary role is to lower blood calcium levels. It does this by inhibiting osteoclast activity, which are the cells responsible for bone resorption, and promoting osteoblast activity, the cells that build new bone.

Increased calcitonin leads to enhanced osteoblast activity, as these cells take up calcium from the bloodstream and use it to create new bone tissue. This results in a net deposition of calcium in the bones, which can contribute to increased bone density over time, although the immediate effect of calcitonin is aimed at lowering blood calcium levels.

Thus, if excessive calcitonin is present, it will not only decrease calcium levels in the blood but also promote the activity of osteoblasts. This action underlines the importance of calcitonin in bone metabolism and calcium homeostasis.