Calcitonin is produced by –
**Core Concept**
Calcitonin is a hormone involved in calcium and phosphorus metabolism. It is secreted in response to elevated blood calcium levels and helps to decrease blood calcium levels by inhibiting osteoclast activity and increasing renal calcium excretion.
**Why the Correct Answer is Right**
Calcitonin is produced by the thyroid gland, specifically by the parafollicular cells, also known as C cells. These cells are distinct from the follicular cells that produce thyroid hormones. Calcitonin plays a crucial role in maintaining calcium homeostasis by counteracting the effects of parathyroid hormone (PTH) and vitamin D. When blood calcium levels rise, calcitonin is released, which then inhibits osteoclast activity, reducing bone resorption and subsequently lowering blood calcium levels.
**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because calcitonin is not produced by the adrenal glands, which produce a variety of hormones such as cortisol and aldosterone.
**Option B:** Incorrect because calcitonin is not produced by the pancreas, which produces hormones such as insulin and glucagon.
**Option C:** Incorrect because calcitonin is not produced by the pituitary gland, which produces hormones such as ACTH and TSH that regulate other endocrine glands.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that calcitonin is used as a diagnostic marker for medullary thyroid carcinoma, a type of thyroid cancer that originates from the parafollicular cells.
**Correct Answer: D. The thyroid gland**