Calcitonin is produced by
**Core Concept**
Calcitonin is a hormone involved in calcium and phosphorus metabolism, playing a crucial role in maintaining serum calcium levels within a narrow range. It is secreted in response to elevated serum calcium levels, acting to decrease osteoclast activity, reduce bone resorption, and increase renal excretion of calcium.
**Why the Correct Answer is Right**
Calcitonin is produced by the parafollicular cells, also known as C cells, of the thyroid gland. When serum calcium levels rise, calcitonin is released, binding to its receptor on osteoclasts, thereby inhibiting their activity and reducing bone resorption. This hormone also decreases renal reabsorption of calcium, promoting its excretion in the urine.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as calcitonin is not produced by the adrenal glands, which are involved in the secretion of other hormones like aldosterone and cortisol.
**Option B:** Calcitonin is not produced by the parathyroid glands, which are responsible for the secretion of parathyroid hormone (PTH), a hormone that increases serum calcium levels.
**Option C:** Calcitonin is not produced by the kidneys, which are involved in the regulation of calcium and phosphorus levels through mechanisms like renal reabsorption and excretion.
**Clinical Pearl / High-Yield Fact**
Remember that calcitonin is used as a biomarker for medullary thyroid carcinoma (MTC), a type of thyroid cancer that arises from the parafollicular cells.
**Correct Answer: C. The parafollicular cells of the thyroid gland.**