What does the parathyroid gland regulate?

The parathyroid gland is primarily responsible for regulating calcium levels in the blood and bones. It produces parathyroid hormone (PTH), which plays a crucial role in maintaining calcium homeostasis. When calcium levels in the blood drop, PTH is released, prompting the bones to release stored calcium, increasing its absorption in the intestines, and enhancing calcium reabsorption in the kidneys. This regulation is vital because calcium is essential for various physiological functions, including muscle contraction, nerve function, and blood clotting.

The other options are associated with different glands or hormonal functions. Insulin production is managed by the pancreas, adrenaline release is primarily controlled by the adrenal glands, and the body's response to stress involves a variety of hormones and systems, including cortisol from the adrenal glands. Thus, the primary regulatory function of the parathyroid gland is indeed focused on calcium management, making it the correct response.