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The hormone that regulates blood calcium levels is parathyroid hormone. This hormone is produced by the parathyroid glands, which are located in the neck near the thyroid gland. Parathyroid hormone plays a crucial role in maintaining calcium homeostasis in the body. It does so by increasing the concentration of calcium in the bloodstream through several mechanisms.

When blood calcium levels drop, parathyroid hormone is released into the bloodstream. It enhances the release of calcium from bones by stimulating the activity of osteoclasts, which are cells that break down bone tissue. Additionally, parathyroid hormone increases the absorption of calcium in the intestines, particularly through its effects on vitamin D metabolism, and it decreases the excretion of calcium in the kidneys. Through these actions, parathyroid hormone successfully raises blood calcium levels back to an optimal range.

In this case, the other hormones mentioned do not primarily regulate blood calcium levels. Insulin is mainly responsible for regulating blood glucose levels. Growth hormone influences growth and metabolism but does not directly control calcium levels. Thyroid hormone has various roles in metabolic regulation but does not specifically target calcium levels as parathyroid hormone does.