What hormone functions to lower blood calcium levels?

The hormone that functions to lower blood calcium levels is calcitonin. Calcitonin is produced by the parafollicular cells (C cells) of the thyroid gland and plays a crucial role in regulating calcium homeostasis in the body. When blood calcium levels rise, calcitonin is secreted to help lower these levels by promoting the deposition of calcium into bones and inhibiting the activity of osteoclasts, the cells responsible for bone resorption. This process effectively reduces the amount of calcium released into the bloodstream.

In contrast, parathyroid hormone is responsible for increasing blood calcium levels by stimulating osteoclast activity, enhancing calcium reabsorption in the kidneys, and promoting the activation of vitamin D to increase intestinal absorption of calcium. Vitamin D itself also functions to increase blood calcium levels by enhancing calcium absorption from the diet. Insulin, primarily known for its role in glucose metabolism, does not have a direct effect on calcium levels.

Thus, calcitonin serves as the primary hormone that actively decreases blood calcium levels, making it the correct answer in this context.