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

• A. Calcitonin
• B. Parathyroid Hormone (PTH)
• C. Vitamin D
• D. Insulin

Answer: (B)

Parathyroid Hormone (PTH) is released in response to low calcium levels in the blood. Its primary function is to help raise blood calcium levels to maintain homeostasis. When calcium levels drop, the parathyroid glands detect this decrease and secrete PTH.

PTH acts on several target organs: it stimulates the release of calcium from bones, increases the reabsorption of calcium in the kidneys, and enhances the activation of vitamin D in the kidneys. The activated vitamin D then facilitates the absorption of calcium from the intestine. Thus, the actions of PTH effectively work to elevate blood calcium levels, counteracting the initial deficiency.

In contrast to PTH, calcitonin, released by the thyroid gland, lowers blood calcium levels when they are elevated. Vitamin D is essential for calcium absorption but does not act directly in response to acute changes in blood calcium levels in the same way PTH does. Insulin, primarily known for its role in glucose metabolism, is not involved in calcium regulation.