What hormone acts antagonistically to calcitonin?

The hormone that acts antagonistically to calcitonin is parathyroid hormone (PTH). Calcitonin, which is produced by the parafollicular cells of the thyroid gland, functions to lower blood calcium levels by inhibiting osteoclast activity, which reduces bone resorption, and promoting the excretion of calcium by the kidneys. In contrast, PTH is produced by the parathyroid glands and has the primary role of increasing blood calcium levels. It achieves this by stimulating osteoclasts to increase bone resorption, enhancing renal tubular reabsorption of calcium, and promoting the conversion of vitamin D to its active form, which increases intestinal absorption of calcium.

Thus, PTH and calcitonin have opposing effects on blood calcium levels, making PTH the correct answer as the hormone that acts antagonistically to calcitonin. Understanding the relationship between these two hormones is critical, as they play significant roles in calcium homeostasis, which is vital for various physiological processes such as muscle contraction, nerve transmission, and blood coagulation.