American Board of Surgery Qualifying Exam (ABS QE) Practice Test

Calcitonin is a hormone produced primarily by the thyroid gland, and its primary action is to decrease osteoclast activity. Osteoclasts are the cells responsible for bone resorption, and when calcitonin is released, it inhibits the activity of these cells. This leads to a reduction in the breakdown of bone tissue, resulting in decreased release of calcium from the bones into the bloodstream. Consequently, this action helps to lower blood calcium levels and maintain calcium homeostasis in the body.

By decreasing osteoclast activity, calcitonin serves as a counter-regulatory mechanism to parathyroid hormone (PTH), which raises blood calcium levels when they are low. Thus, the function of calcitonin is essential in the regulation of calcium metabolism, particularly during periods when calcium levels increase, such as after a calcium-rich meal.

The other options do not accurately reflect the primary action of calcitonin. While calcitonin may have some influence on calcium absorption in the intestines, its primary role is more directly linked to the actions within bone tissue and its regulatory effects on osteoclasts. Additionally, calcitonin does not increase kidney reabsorption of calcium; instead, it generally contributes to the excretion of calcium through the kidneys