Which hormone is primarily responsible for regulating calcium levels in the body?

The hormone that is primarily responsible for regulating calcium levels in the body is Parathyroid Hormone (PTH). PTH is secreted by the parathyroid glands and plays a critical role in maintaining calcium homeostasis. When calcium levels in the blood drop, PTH is released and acts on various organs to increase calcium levels. It stimulates the release of calcium from bones into the bloodstream, enhances the reabsorption of calcium in the kidneys, and promotes the conversion of vitamin D into its active form, which increases calcium absorption from the intestines. This coordinated action helps to elevate blood calcium levels to the necessary range for normal physiological function.

Insulin is involved in glucose metabolism and does not play a direct role in calcium regulation. Thyroxine, a thyroid hormone, primarily regulates metabolism and energy levels, while glucagon is involved in raising blood glucose levels. None of these hormones have the regulatory role over calcium that PTH does, making PTH the correct answer for this question.