Final answer:
Calcitonin is the hormone made by the parafollicular cells (C cells) of the thyroid gland, and it helps regulate calcium concentrations in the body.
Step-by-step explanation:
The hormone that is made by the parafollicular cells of the thyroid gland is calcitonin. These cells, also known as C cells, produce calcitonin in response to high levels of calcium in the blood. This hormone helps to regulate calcium concentrations in body fluids by inhibiting osteoclast activity in the bones and stimulating the excretion of calcium in the kidneys. Unlike other thyroid hormones, calcitonin is not controlled by thyroid-stimulating hormone (TSH) but is stimulated by the calcium ion concentrations in the blood directly.