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

The hormone that is secreted in response to low blood calcium levels is parathyroid hormone (PTH). When blood calcium levels drop, the parathyroid glands, which are located on the posterior aspect of the thyroid gland, release PTH into the bloodstream. This hormone plays a crucial role in regulating calcium levels by:

1. Increasing the release of calcium from bone tissue into the bloodstream, thereby raising blood calcium levels.

2. Enhancing the reabsorption of calcium in the kidneys, which reduces calcium loss in urine.

3. Stimulating the activation of vitamin D in the kidneys, leading to increased intestinal absorption of calcium from the diet.

Together, these actions help restore and maintain normal blood calcium levels, which are essential for various physiological processes, including muscle contraction and nerve function.

In contrast, calcitonin, which is secreted by the thyroid gland, has the opposite effect by lowering blood calcium levels when they are elevated. Insulin and glucagon, hormones produced by the pancreas, are primarily involved in glucose metabolism and have no direct role in calcium homeostasis.