Final answer:
Calcitonin, produced by the thyroid gland, functions to decrease the level of calcium ions in the blood by inhibiting osteoclast activity, stimulating osteoblasts, and increasing calcium excretion by the kidneys.
Step-by-step explanation:
The hormone calcitonin is produced by the parafollicular cells of the thyroid gland and serves to regulate calcium levels in the blood. Among its functions, calcitonin decreases blood calcium levels by inhibiting the activity of osteoclasts, stimulating the growth and activity of osteoblasts, and enhancing the excretion of calcium through the kidneys. Therefore, the primary action of calcitonin is to decrease the level of calcium ions in the blood by these mechanisms, which results in calcium being deposited in the bones to promote structural integrity and bone growth.