What hormone is released in response to low calcium levels?

Parathyroid hormone (PTH) is released by the parathyroid glands in response to low calcium levels in the blood, playing a critical role in maintaining calcium homeostasis. When calcium levels decrease, sensors in the parathyroid glands detect this change and prompt the secretion of PTH. This hormone acts to increase calcium levels through various mechanisms:

1. It stimulates the release of calcium from bones by promoting osteoclast activity, which breaks down bone tissue and releases calcium into the bloodstream.

2. PTH enhances renal tubular reabsorption of calcium, reducing the amount of calcium excreted in urine while simultaneously promoting phosphate excretion.

3. It also increases the activation of vitamin D in the kidneys, which in turn enhances intestinal absorption of calcium from dietary sources.

The other choices do not function to directly raise calcium levels in the same manner. Calcitonin, for instance, lowers blood calcium levels by inhibiting bone resorption and promoting calcium excretion in the kidneys. Vitamin D is important for calcium absorption but is activated by PTH rather than being released in response to low calcium levels. Aldosterone primarily regulates sodium and potassium balance, not calcium levels. Thus, PTH is the primary hormone responsible for counteracting low calcium levels in