Final answer:
The hormone that inhibits osteoclast activity when blood calcium levels are high is calcitonin, which lowers the calcium levels by favoring bone formation and reducing kidneys' reabsorption of calcium.
Step-by-step explanation:
When blood calcium levels rise above normal, osteoclast activity is inhibited by the hormone calcitonin. Calcitonin is released by the thyroid gland and works to decrease blood calcium levels through various mechanisms. It inhibits the activity of osteoclasts, which are cells responsible for breaking down bone and releasing calcium into the bloodstream. Furthermore, calcitonin stimulates calcium uptake by the bones and decreases the reabsorption of calcium by the kidneys, ultimately lowering blood calcium levels. Unlike calcitonin, parathyroid hormone (PTH) actually increases blood calcium levels when they are low, by stimulating osteoclasts and enhancing the absorption of dietary calcium with the help of active vitamin D.