The relationship between serum intact parathyroid hormone and calcium in idiopathic hypoparathyroidism.

We studied the relationship between serum calcium (Ca) and parathyroid hormone (PTH) in patients with idiopathic hypoparathyroidism (IHP) to investigate the diversity of the degree of parathyroid insufficiency and a regulation of PTH secretion by Ca in IHP. Serum intact PTH concentrations ranged from < 1.0 to 10.3 pg/ml in 20 patients with IHP and were above the detection limit in 18 (90%) patients. Serum total Ca levels in IHP were positively correlated with serum intact PTH (r = 0.824, P < 0.001), in contrast with a negative correlation between serum intact PTH and Ca in 91 normal subjects (r = -0.378, P < 0.001) and abnormally high serum intact PTH values against hypocalcemia in five patients with pseudohypoparathyroidism. In three patients with IHP, serum intact PTH decreased in response to treatment with active vitamin D3 which increased serum Ca and 1,25(OH)2D. An inverse sigmoidal relationship between serum ionized Ca and intact PTH was demonstrated in a patient with IHP before treatment [IHP(-)] and three patients with IHP treated with active vitamin D3[IHP(+)]. Peak serum intact PTH values were reduced in IHP(-) and IHP(+). Baseline serum intact PTH was 77.0% of the maximal level in IHP(-), 51.7-72.1% in IHP(+), and 31.1-42.3% in five normal subjects. The degree of parathyroid dysfunction in IHP differs from one patient to another and serum Ca levels are thought to be determined by circulating PTH levels. Although PTH secretory reserve is diminished, PTH secretion is negatively regulated by Ca and is stimulated at baseline in patients with IHP before and during the treatment with active vitamin D3.