Which of the following conditions is associated with high PTH and low calcium levels?

• A. Hypercalcemia
• B. Vitamin D deficiency
• C. Hyperparathyroidism
• D. Acidosis

Answer: (B)

The association of high parathyroid hormone (PTH) levels with low calcium levels typically indicates a response to low serum calcium rather than hyperparathyroidism itself, where PTH would normally be high alongside elevated calcium levels. Vitamin D deficiency is a known condition that can lead to low calcium levels due to inadequate absorption of calcium from the diet. Consequently, in the absence of sufficient Vitamin D, the body may respond by increasing PTH levels in an effort to maintain calcium homeostasis, even when calcium levels are still low.

In this context, the body is attempting to compensate for the low calcium levels by secreting more PTH, which usually acts to raise calcium levels by mobilizing it from bone and enhancing renal tubular reabsorption of calcium. However, if Vitamin D levels are inadequate, this compensatory mechanism can be insufficient, leading to high PTH but persistently low calcium levels.

The other conditions listed do not align with this specific biochemical profile. Hypercalcemia would not show high PTH levels, as high calcium typically suppresses PTH. Hyperparathyroidism usually results in high PTH levels alongside high calcium levels due to the overproduction of PTH. Acidosis may affect calcium levels, but it does not characteristically