Which hormone helps to regulate blood calcium levels, often opposing the actions of parathyroid hormone?

Calcitonin is the hormone that plays a critical role in regulating blood calcium levels, primarily by opposing the effects of parathyroid hormone (PTH). When calcium levels in the blood rise, calcitonin is secreted by the parafollicular cells (C cells) of the thyroid gland. Its primary function is to lower blood calcium levels by inhibiting osteoclast activity in the bones. Osteoclasts are responsible for breaking down bone tissue and releasing calcium into the bloodstream. By reducing the activity of these cells, calcitonin helps to deposit calcium back into the bone, leading to a decrease in circulating calcium levels.

Additionally, calcitonin can enhance the renal excretion of calcium, further contributing to the decrease in blood calcium concentration. In contrast, parathyroid hormone increases blood calcium levels by stimulating osteoclasts, promoting calcium reabsorption in the kidneys, and increasing intestinal absorption of calcium. Therefore, calcitonin’s role as a counter-regulatory hormone to PTH is crucial for maintaining calcium homeostasis in the body.