Which hormone increases blood calcium levels when it is secreted?

• A. Calcitonin
• B. Insulin
• C. Aldosterone
• D. Parathyroid hormone (PTH)

Answer: (D)

Verified step by step guidance

1. Step 1: Understand the role of calcium in the body. Calcium is essential for various physiological processes, including muscle contraction, nerve signaling, and blood clotting. The body tightly regulates blood calcium levels to maintain homeostasis.
2. Step 2: Identify the hormones involved in calcium regulation. The key hormones are Parathyroid Hormone (PTH), Calcitonin, and Vitamin D. PTH increases blood calcium levels, Calcitonin decreases blood calcium levels, and Vitamin D aids in calcium absorption from the intestines.
3. Step 3: Learn the mechanism of Parathyroid Hormone (PTH). PTH is secreted by the parathyroid glands in response to low blood calcium levels. It acts on three main targets: bones (stimulates osteoclasts to release calcium into the blood), kidneys (reduces calcium excretion and increases phosphate excretion), and intestines (indirectly increases calcium absorption by activating Vitamin D).
4. Step 4: Compare PTH with Calcitonin. Calcitonin, secreted by the thyroid gland, has the opposite effect of PTH. It lowers blood calcium levels by inhibiting osteoclast activity and promoting calcium storage in bones. This distinction helps clarify why PTH increases blood calcium levels.
5. Step 5: Eliminate the incorrect options. Insulin regulates blood glucose levels, not calcium. Aldosterone regulates sodium and potassium balance, not calcium. Calcitonin decreases blood calcium levels, making Parathyroid Hormone (PTH) the correct answer.