Which hormone opposes the action of parathyroid hormone (PTH) in regulating blood calcium levels?

• A. Insulin
• B. Calcitonin
• C. Cortisol
• D. Aldosterone

Answer: (B)

Verified step by step guidance

1. Step 1: Understand the role of parathyroid hormone (PTH) in regulating blood calcium levels. PTH increases blood calcium levels by stimulating bone resorption (breaking down bone tissue to release calcium), increasing calcium absorption in the intestines, and reducing calcium excretion by the kidneys.
2. Step 2: Identify the hormone that has the opposite effect of PTH. This hormone should decrease blood calcium levels by promoting calcium deposition in bones and reducing calcium levels in the bloodstream.
3. Step 3: Review the function of calcitonin. Calcitonin is a hormone produced by the thyroid gland that lowers blood calcium levels by inhibiting bone resorption and promoting calcium storage in bones.
4. Step 4: Compare the functions of the other hormones listed (Insulin, Cortisol, Aldosterone) to determine if they regulate blood calcium levels. Insulin regulates blood glucose, Cortisol is involved in stress response and metabolism, and Aldosterone regulates sodium and potassium balance.
5. Step 5: Conclude that calcitonin is the hormone that opposes the action of PTH in regulating blood calcium levels, as it decreases blood calcium levels while PTH increases them.