Which hormone is secreted by the thyroid gland?

• A. Calcitonin
• B. PTH
• C. Insulin
• D. Adrenaline

Answer: (A)

Calcitonin is the hormone secreted by the thyroid gland. It is produced by the parafollicular cells (C cells) in the thyroid and helps lower blood calcium levels when they are high, mainly by inhibiting bone breakdown (osteoclast activity) and promoting calcium excretion in the kidneys. The other hormones come from different glands: PTH is made by the parathyroid glands and raises blood calcium; insulin is produced by the pancreas and regulates blood glucose; adrenaline (epinephrine) comes from the adrenal medulla and mediates rapid sympathetic responses. The thyroid does secrete other hormones as well, like T3 and T4, but the option that is a hormone produced by the thyroid gland in this context is calcitonin.