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

The hormone primarily responsible for regulating calcium levels in the blood is parathyroid hormone (PTH). PTH is secreted by the parathyroid glands and plays a crucial role in maintaining calcium homeostasis by increasing serum calcium levels when they drop too low. It achieves this through several mechanisms: stimulating the release of calcium from bones, increasing the reabsorption of calcium in the kidneys, and promoting the conversion of vitamin D into its active form, which enhances intestinal absorption of calcium.

In contrast, calcitonin, produced by the thyroid gland, acts to lower blood calcium levels, but its effect is secondary and does not play a central role in calcium regulation compared to PTH. Thyroid hormones are primarily involved in regulating metabolism and do not significantly influence calcium levels. Insulin is a hormone involved in glucose metabolism and has no direct effect on calcium regulation. Thus, parathyroid hormone is the key regulator of calcium levels in the blood.