The Hormone inhibiting both resorption and formation of bone
**Core Concept**
The question targets the understanding of hormones regulating bone metabolism. Parathyroid hormone (PTH) primarily stimulates bone resorption, while thyroid hormone (TH) generally enhances bone formation. However, another hormone counteracts both processes.
**Why the Correct Answer is Right**
The hormone inhibiting both bone resorption and formation is calcitonin. Calcitonin is a peptide hormone produced by the thyroid gland, which plays a crucial role in calcium homeostasis. It acts on osteoclasts to decrease bone resorption and on osteoblasts to decrease bone formation. This dual effect helps to lower blood calcium levels when they are elevated. Calcitonin exerts its effects through the G-protein coupled calcitonin receptor (CTR), which is expressed on osteoclasts and osteoblasts.
**Why Each Wrong Option is Incorrect**
* **Option A:** Parathyroid hormone (PTH) primarily stimulates bone resorption and formation, but its net effect is to increase blood calcium levels. It acts through the PTH receptor (PTHR), which is expressed on osteoclasts and osteoblasts.
* **Option B:** Thyroid hormone (TH) generally enhances bone formation, but it has a minimal effect on bone resorption. TH acts through thyroid hormone receptors (THRA and THRB), which are expressed on osteoblasts.
* **Option C:** Estrogen is a hormone that inhibits bone resorption but stimulates bone formation. Estrogen acts through estrogen receptors (ESR1 and ESR2), which are expressed on osteoclasts and osteoblasts.
**Clinical Pearl / High-Yield Fact**
Calcitonin is a hormone that counteracts the effects of parathyroid hormone (PTH) and thyroid hormone (TH) on bone metabolism. It is often used in the treatment of hypercalcemia and osteoporosis.
**Correct Answer:** . Calcitonin