Which substance has a primary role in increasing osteoclast activity?

Parathyroid hormone plays a crucial role in regulating calcium levels in the body, particularly by increasing osteoclast activity. Osteoclasts are specialized cells responsible for bone resorption, which is the process of breaking down bone tissue to release calcium and phosphate into the bloodstream. When parathyroid hormone is secreted in response to low blood calcium levels, it stimulates osteoclasts, leading to increased bone resorption and consequently raising blood calcium levels.

This action is vital for maintaining calcium homeostasis, which is essential for various physiological functions, including muscle contraction, nerve signaling, and blood coagulation. By increasing osteoclast activity, parathyroid hormone ensures that the body can quickly mobilize calcium when needed, such as during periods of low dietary intake or increased physiological demand.

The other substances mentioned, such as calcitonin, insulin, and cortisol, do not primarily increase osteoclast activity. Calcitonin actually works to lower blood calcium levels by inhibiting osteoclast function; insulin is involved in glucose metabolism and does not directly affect osteoclast activity; and while cortisol can influence bone metabolism, its primary function is related to stress responses and metabolism rather than directly increasing osteoclast activity.