In primary hypoparathyroidism, which statement best describes serum phosphate levels?

In primary hypoparathyroidism, which statement best describes serum phosphate levels?

• A. Low phosphate
• B. Slightly elevated phosphate
• C. Normal phosphate
• D. High phosphate

Answer: (D)

The key concept is how parathyroid hormone (PTH) regulates phosphate handling by the kidney. PTH lowers serum phosphate by promoting its excretion in the proximal tubule. When PTH is absent or very low, the kidney reabsorbs more phosphate, so phosphate accumulates in the blood. In primary hypoparathyroidism, there isn’t enough PTH to drive that phosphaturic effect, so phosphate reabsorption in the proximal tubule increases and serum phosphate rises. This typically presents alongside hypocalcemia, since PTH also normally raises calcium by increasing bone resorption and renal calcium reabsorption. So, the best description of serum phosphate in primary hypoparathyroidism is that it is high. By contrast, conditions with excess PTH would more often show low or inappropriately normal phosphate due to increased renal phosphate wasting.

The key concept is how parathyroid hormone (PTH) regulates phosphate handling by the kidney. PTH lowers serum phosphate by promoting its excretion in the proximal tubule. When PTH is absent or very low, the kidney reabsorbs more phosphate, so phosphate accumulates in the blood.

In primary hypoparathyroidism, there isn’t enough PTH to drive that phosphaturic effect, so phosphate reabsorption in the proximal tubule increases and serum phosphate rises. This typically presents alongside hypocalcemia, since PTH also normally raises calcium by increasing bone resorption and renal calcium reabsorption.

So, the best description of serum phosphate in primary hypoparathyroidism is that it is high. By contrast, conditions with excess PTH would more often show low or inappropriately normal phosphate due to increased renal phosphate wasting.