Which hormone is released in response to low blood calcium levels?

Parathyroid hormone (PTH) is released in response to low blood calcium levels, playing a crucial role in maintaining calcium homeostasis in the body. When blood calcium levels drop, the parathyroid glands sense this decrease and secrete PTH. The hormone acts on various target tissues to increase blood calcium levels through several mechanisms:

1. Bone: PTH stimulates osteoclast activity, which leads to the resorption of bone tissue and the release of calcium into the bloodstream.
2. Kidneys: PTH promotes calcium reabsorption in the renal tubules, reducing the amount of calcium excreted in urine. It also stimulates the conversion of vitamin D to its active form (calcitriol) in the kidneys.

3. Intestines: The active form of vitamin D enhances intestinal absorption of calcium from the diet.

The net effect of PTH is an increase in serum calcium levels, counteracting the initial deficiency.

In contrast, calcitonin, released by the thyroid gland, functions to lower blood calcium levels and is not a response to low levels. Insulin and glucagon are primarily hormones involved in glucose metabolism and do not have a direct role in calcium homeostasis.