Which statement regarding calcitonin is correct?

The statement that calcitonin's actions are opposite to that of parathyroid hormone is correct. Calcitonin is a hormone produced by the parafollicular cells (also known as C cells) of the thyroid gland. Its primary role is to lower blood calcium levels, which it achieves by inhibiting osteoclast activity in the bones, leading to decreased bone resorption and subsequent release of calcium into the bloodstream. On the other hand, parathyroid hormone (PTH) has the opposite effect; it raises blood calcium levels by stimulating osteoclasts, promoting bone resorption, and increasing calcium reabsorption in the kidneys. Therefore, calcitonin functions in a regulatory capacity, acting to counterbalance the effects of PTH in maintaining calcium homeostasis within the body.

Other statements do not accurately reflect the functions and characteristics of calcitonin. For instance, calcitonin is not secreted by follicular cells, it does not decrease phosphorus levels by increasing bone resorption, and while thyroid hormone plays a role in metabolism and has some influence on calcium metabolism, calcitonin primarily operates as a counter-regulatory hormone to maintain serum calcium levels.