Correct Option (C)
Parathyroid hormone (PTH) is secreted by the parathyroid glands. It is the primary hormone responsible for regulating blood calcium and phosphate homeostasis. PTH increases blood calcium levels by stimulating the release of calcium from bones (bone resorption), enhancing the reabsorption of calcium in the renal tubules, and promoting the synthesis of calcitriol (active vitamin D) in the kidneys, which subsequently increases intestinal calcium absorption. Concurrently, PTH lowers blood phosphate levels by reducing its reabsorption in the renal tubules, thereby increasing its excretion.
Incorrect Options:
Glucagon is a hormone secreted by the alpha cells of the pancreas. Its primary role is to raise blood glucose levels by promoting glycogenolysis and gluconeogenesis in the liver. It does not regulate blood calcium or phosphate.
Growth hormone, secreted by the anterior pituitary gland, primarily stimulates growth, cell reproduction, and regeneration. While it has metabolic effects, it does not directly regulate blood calcium or phosphate levels.
Thyroxine (T4) is a hormone produced by the thyroid gland. Its main function is to regulate the body's metabolic rate, influencing growth, development, and energy expenditure. It is not involved in the regulation of blood calcium or phosphate balance.