Parathormone secretion is stimulated by ?
**Core Concept**
Parathormone (PTH) secretion is a critical regulatory mechanism that maintains calcium homeostasis in the body. The parathyroid glands, located near the thyroid gland, produce PTH in response to low calcium levels in the blood. This hormone plays a pivotal role in stimulating calcium release from bones, increasing calcium absorption from the gut, and promoting calcium reabsorption in the kidneys.
**Why the Correct Answer is Right**
PTH secretion is primarily stimulated by low calcium levels in the blood, which activate the calcium-sensing receptors on the surface of parathyroid chief cells. This activation triggers a signaling cascade that ultimately leads to the release of PTH from the parathyroid glands. The correct answer is a hormone that is known to decrease calcium levels in the blood, thereby stimulating PTH secretion.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because glucagon, a hormone produced by the pancreas, actually increases blood calcium levels by stimulating the release of calcium from stored sites and increasing calcium absorption from the gut.
**Option B:** This option is incorrect because insulin, a hormone produced by the pancreas, also increases blood calcium levels by stimulating the release of calcium from stored sites and increasing calcium absorption from the gut.
**Option C:** This option is incorrect because calcitonin, a hormone produced by the thyroid gland, actually decreases blood calcium levels by stimulating the deposition of calcium in bones and increasing calcium excretion in the urine.
**Clinical Pearl / High-Yield Fact**
A key point to remember is that PTH secretion is primarily regulated by the calcium-sensing receptors on the surface of parathyroid chief cells, which respond to changes in blood calcium levels to maintain calcium homeostasis.
**Correct Answer:** Calcium.