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

The hormone produced in response to low blood calcium levels is parathyroid hormone (PTH). When calcium levels in the blood decrease, the parathyroid glands sense this drop and secrete PTH. PTH acts on various targets in the body to restore calcium levels to the normal range.

Primarily, PTH increases calcium levels by promoting the release of calcium from bones, enhancing intestinal absorption of calcium (through its activation of vitamin D), and increasing renal tubular reabsorption of calcium, reducing the amount of calcium excreted in urine. Thus, the secretion of PTH plays a crucial role in maintaining calcium homeostasis.

The other hormones mentioned do not function to increase blood calcium levels in response to low calcium. For instance, calcitonin is involved in lowering blood calcium levels when they are high. Vitamin D, while it aids in calcium absorption, is not released directly in response to low calcium but is activated by PTH. Thyroid hormone does not play a primary role in calcium homeostasis. Hence, PTH is the key hormone responsible for increasing blood calcium levels when they fall.