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

Calcitonin is a hormone produced by the parafollicular cells (C cells) of the thyroid gland and plays a crucial role in calcium homeostasis in the body. Its primary function is to lower blood calcium levels when they become elevated. It accomplishes this by directly inhibiting the activity of osteoclasts, which are the bone cells responsible for the resorption of bone tissue. By reducing osteoclast activity, calcitonin effectively decreases the release of calcium from bones into the bloodstream, thereby lowering blood calcium levels. In contrast, parathyroid hormone increases osteoclast activity to raise blood calcium levels. Insulin primarily impacts glucose metabolism and does not have a direct effect on osteoclasts. Cortisol has a complex role in bone metabolism, generally leading to increased osteoclast activity over time, which can contribute to bone loss when present in excess. The action of calcitonin in reducing osteoclast activity makes it a key player in the regulation of calcium homeostasis and bone density, highlighting its importance in maintaining skeletal health and preventing conditions such as osteoporosis.