Final answer:
Calcitonin, produced by the thyroid gland's C cells, lowers blood calcium levels by inhibiting osteoclasts and stimulating osteoblasts, and it is secreted in response to high blood calcium levels.
Step-by-step explanation:
Calcitonin is a peptide hormone produced and secreted by the parafollicular cells (C cells) of the thyroid gland. Its primary role is to lower blood calcium levels (Ca²⁺).
Calcitonin is secreted in response to high concentrations of calcium in the blood; it functions by inhibiting osteoclasts, which break down bone, and by stimulating the activity of osteoblasts, which build up the bone. Additionally, calcitonin enhances the excretion of calcium through the kidneys.
These actions help move calcium from the blood and deposit it into the bones, thereby preserving bone strength and structural integrity.
Calcitonin plays an important role in maintaining calcium homeostasis in coordination with parathyroid hormone (PTH), which has the opposite effect. It is particularly significant during periods of growth, pregnancy, and starvation.