What hormone increases blood calcium levels?

The hormone that increases blood calcium levels is parathyroid hormone (PTH). PTH is secreted by the parathyroid glands, which are located near the thyroid gland in the neck. The primary function of PTH is to maintain calcium homeostasis in the body. It achieves this by stimulating the release of calcium from the bones, increasing calcium reabsorption in the kidneys, and enhancing the activation of vitamin D in the kidneys, which in turn promotes intestinal absorption of calcium.

This intricate regulation is crucial, as maintaining appropriate calcium levels is vital for various physiological processes, including muscle contraction, blood clotting, and nerve function. In contrast, calcitonin works to lower blood calcium levels, while insulin and estrogen have different roles in metabolism and hormonal regulation rather than directly affecting calcium levels in the blood. Thus, parathyroid hormone is the key player in increasing blood calcium levels, making it the correct answer.