What hormone is secreted by the parafollicular cells of the thyroid?

The correct hormone secreted by the parafollicular cells of the thyroid is calcitonin. Parafollicular cells, also known as C cells, are located in the thyroid gland and play a vital role in calcium homeostasis. Calcitonin's primary function is to lower blood calcium levels by inhibiting osteoclast activity in the bones, which reduces the release of calcium from bone into the bloodstream, and it also decreases renal tubular reabsorption of calcium, promoting its excretion in urine.

The other hormones mentioned are produced by different cells or glands. Thyroxine and triiodothyronine are secreted by the follicular cells of the thyroid and are involved in regulating metabolism. Parathyroid hormone, on the other hand, is secreted by the parathyroid glands and acts to increase blood calcium levels, opposing the action of calcitonin. This delineation of functions and origins highlights why calcitonin is the accurate answer in this context.