Which hormone is produced by the C-cells of the thyroid gland and serves to decrease serum calcium levels?

The correct response highlights the role of calcitonin, a hormone produced by the C-cells (or parafollicular cells) of the thyroid gland. Calcitonin is vital for regulating calcium levels in the bloodstream. It functions primarily through its ability to decrease serum calcium levels in two primary ways: first, by inhibiting osteoclast activity in the bones, which are the cells responsible for the breakdown of bone tissue and release of calcium into the bloodstream; and second, by decreasing renal tubular reabsorption of calcium, leading to increased excretion of calcium in the urine.

Understanding this role is essential in comprehending calcium homeostasis in the body. The balance of calcium levels is critical for numerous physiological functions, including muscle contraction, nerve function, and blood coagulation. Calcitonin acts counter to parathyroid hormone (PTH), which increases blood calcium levels when they fall below optimal levels.

In contrast, PTH, calcitriol (active form of vitamin D that promotes calcium absorption from the intestine), and cortisol (a glucocorticoid hormone involved in metabolism and stress response) do not have the same function in lowering serum calcium. PTH specifically raises calcium levels, while calcitriol further promotes increases and is associated