Laboratory Findings in Hypoparathyroidism

The serum calcium level is low (5–7 mg/dL), and the phosphorus level is elevated (7–12 mg/dL).

Blood levels of ionized calcium (usually approximately 45% of the total) more nearly reflect physiologic adequacy but also are low.

The serum level of alkaline phosphatase is normal or low, and the level of 1,25[OH]2D3 is usually low, but high levels have been found in some children with severe hypocalcemia.

The level of magnesium is normal but should always be checked in hypocalcemic patients.

Levels of PTH are low when measured by immunometric assay.

Administration of the synthetic 1–34 fragment of human PTH (teriparatide acetate) results in increased urinary levels of cyclic adenosine monophosphate and phosphate. This response differentiates hypoparathyroidism from pseudohypoparathyroidism. With the advent of very sensitive PTH assays, this test is usually not necessary.

Radiographs of the bones occasionally reveal an increased density limited to the metaphyses, suggestive of heavy metal poisoning, or an increased density of the lamina dura. Radiographs or CT scans of the skull may reveal calcifications in the basal ganglia.

There is a prolongation of the QT interval on the electrocardiogram, which disappears when the hypocalcemia is corrected.

The electroencephalogram usually reveals widespread slow activity; the tracing returns to normal after the serum calcium concentration has been within the normal range for a few weeks, unless irreversible brain damage has occurred or unless the parathyroid insufficiency is associated with epilepsy.

When hypoparathyroidism occurs concurrently with Addison disease, the serum level of calcium may be normal, but hypocalcemia appears after effective treatment of the adrenal insufficiency.