Response of plasma 1,25-dihydroxyvitamin D in the human PTH(1-34) infusion test: an improved index for the diagnosis of idiopathic hypoparathyroidism and pseudohypoparathyroidism.

Synthetic human parathyroid hormone (1-34) (hPTH(1-34] infusion test has been utilized in the differential diagnosis of hypoparathyroidism by examining the incremental response of urinary phosphate and cyclic adenosine monophosphate (AMP). The response of plasma levels of 1,25-dihydroxyvitamin D (1,25(OH)2D) in parathyroid hormone (PTH) infusion test was studied as a new criterion for the differential diagnosis of idiopathic hypoparathyroidism (IHP) and pseudohypoparathyroidism (PHP). Fourteen patients with IHP, 4 patients with PHP, and five control subjects were studied. All subjects received an intravenous infusion of 30 micrograms hPTH(1-34) over 5 minutes. The basal levels of plasma 1,25(OH)2D in patients with IHP and PHP were significantly lower than those in control subjects, but there was no significant difference between the levels in patients with IHP and in patients with PHP. The plasma levels of 1,25(OH)2D increased after the infusion of hPTH(1-34) and reached a peak 6 to 24 hours afterward. The 1,25(OH)2D increase at 24 hours afterward the infusion (delta 1,25(OH)2D) in control subjects and in patients with IHP were 18.1 +/- 3.91 (mean +/- SEM) and 24.1 +/- 2.80 pg/ml, respectively. There was no significant increase in patients with PHP (delta 1,25(OH)2D = 4.9 +/- 1.97 pg/ml). From these results, the measurement of delta 1,25(OH)2D in hPTH(1-34) infusion test is useful as a criterion for the differential diagnosis of hypoparathyroidism.