What does the parathyroid hormone primarily regulate in the body?

The parathyroid hormone (PTH) primarily regulates calcium levels in the body, making the chosen answer accurate. PTH, which is secreted by the parathyroid glands, plays a critical role in maintaining calcium homeostasis. When calcium levels in the blood are low, PTH is released, prompting the bones to release calcium, the kidneys to reabsorb more calcium from urine, and the intestines to absorb calcium from food by influencing the production of active vitamin D. This multifaceted approach ensures that blood calcium levels remain within the narrow range necessary for various physiological functions, including muscle contraction, blood clotting, and nerve transmission. The other options are not directly linked to the function of parathyroid hormone; for instance, blood glucose levels are primarily regulated by insulin and glucagon from the pancreas, while body temperature and fluid balance involve different regulatory systems within the body.