Which hormone is primarily responsible for decreasing blood calcium levels?

Calcitonin is the hormone that primarily works to decrease blood calcium levels. It is produced by the parafollicular cells (also known as C cells) of the thyroid gland. When blood calcium levels rise, calcitonin is secreted into the bloodstream, where it promotes the deposition of calcium in the bones and reduces the renal reabsorption of calcium, leading to an increase in calcium excretion in urine. This action helps to lower the overall concentration of calcium in the bloodstream effectively.

In contrast, calcitriol is the active form of vitamin D, and it plays a role in increasing blood calcium levels by promoting intestinal absorption of calcium and reabsorption from the kidneys. Parathyroid hormone serves to increase blood calcium levels by stimulating bone resorption, intestinal absorption, and renal tubular reabsorption of calcium. Cortisol, a glucocorticoid hormone, has complex effects on calcium metabolism but is not directly involved in the regulation of calcium levels in the same manner as calcitonin.