Which statement best describes the regulatory role of parathyroid hormone?

• A. PTH can increase serum calcium levels
• B. PTH decreases serum calcium levels
• C. PTH has no effect on calcium
• D. PTH only affects phosphate

Answer: (A)

Parathyroid hormone’s role is to raise serum calcium when levels are low. It accomplishes this through three main actions: stimulating bone resorption to release calcium into the bloodstream, increasing calcium reabsorption in the kidneys, and promoting activation of vitamin D so the gut can absorb more calcium. It also lowers phosphate reabsorption in the kidneys, promoting phosphate excretion to help balance minerals. Taken together, these effects raise serum calcium, which is why this statement is the best description of PTH’s regulatory role. The idea that PTH decreases calcium, has no effect on calcium, or affects only phosphate doesn’t fit with its calcium-raising actions and broader influence on mineral metabolism.