Which of following laboratory results indicates hypoparathyroidism?

Correct Answer: C. Serum phosphorus level of 5.7 mg/dL.

The parathyroid is responsible for the absorption of calcium and phosphorus. When a client has hypoparathyroidism, the serum calcium levels are low and the serum phosphorus levels are high. The normal phosphorus level is 2.7 to 4.5 mg/dL. Parathyroid hormone deficiency, also called hypoparathyroidism, results in hypocalcemia, hyperphosphatemia, and increased neuromuscular irritability. Patients may present with myalgias, muscle spasms, and in extreme cases tetany.

• Option A: Calcium is maintained within a fairly narrow range from 8.5 to 10.5 mg/dl (4.3 to 5.3 mEq/L or 2.2 to 2.7 mmol/L). Normal values and reference ranges may vary among laboratories as much as 0.5 mg/dl. Aldinger KA, et al., studied a large group of patients of normal renal function with hypercalcemia to determine the prevalence of hypokalemia and reported that 16.9% had hyperparathyroidism, and the degree and frequency of hypokalemia were greatest at the higher serum calcium levels.
• Option B: Parathyroid hormone activates the PTH receptor, another G-protein coupled receptor, increasing resorption of calcium and phosphorus from bone, enhancing the distal tubular reabsorption of calcium, and decreasing the renal tubular reabsorption of phosphorus. Deficient PTH results in hypocalcemia, hyperphosphatemia, while alkaline phosphatase, a marker of bone formation, is normal.
• Option D: The normal range for blood magnesium level is 1.7 to 2.2 mg/dL (0.85 to 1.10 mmol/L). Another common cause of hypoparathyroidism is abnormally low levels of magnesium (hypomagnesemia) in the blood. This is often called functional hypoparathyroidism because it resolves when magnesium is restored. Magnesium is a mineral that is very important in the function of the parathyroid glands.