How does pseudohypoparathyroidism differ from true hypoparathyroidism?

• A. Pseudohypoparathyroidism shows PTH resistance with elevated PTH and low calcium.
• B. Pseudohypoparathyroidism presents with low PTH and low calcium.
• C. True hypoparathyroidism shows PTH resistance with elevated calcium.
• D. True hypoparathyroidism shows high PTH and high calcium.

Answer: (A)

The key idea is distinguishing PTH action from PTH production. In pseudohypoparathyroidism, the parathyroid glands produce PTH, but the body's tissues are resistant to its effects. Because of this end-organ resistance, calcium remains low despite high PTH levels, and the parathyroids respond by raising PTH even more. This pattern—hypocalcemia with inappropriately elevated PTH due to PTH resistance—is what set pseudohypoparathyroidism apart from true hypoparathyroidism.

In true hypoparathyroidism, the problem is insufficient PTH production. Calcium falls because there isn’t enough PTH to boost calcium release from bone, decrease renal phosphate reabsorption, and activate vitamin D; PTH levels are low or absent. So calcium is low, but PTH is not elevated.

That’s why the statement describing pseudohypoparathyroidism as a PTH-resistant state with elevated PTH and low calcium best captures the distinction.