Moderate hypocalcemia due to normal serum 1,25-dihydroxyvitamin D levels in an asymptomatic kindred with familial hypoparathyroidism.

Hypoparathyroidism was diagnosed in nine members of a kindred of three generations. This study investigated why these persons were asymptomatic and without developmental abnormalities, in contrast to the common presentation of idiopathic hypoparathyroidism. In the hypocalcemic subjects, serum calcium level was 7.4 +/- 0.8 mg/dl (mean +/- SD) and ionized serum calcium level was 3.48 +/- 0.21 mg/dl. Immunoreactive parathyroid hormone values were inappropriately low. Injection of EDTA in one patient lowered ionized calcium levels, but immunoreactive parathyroid hormone values did not rise. Serum levels of 1,25-dihydroxyvitamin D and other vitamin D metabolites were normal or elevated and substantially higher than in other hypoparathyroid states. The normally observed positive correlation between the fasting urinary calcium/creatinine ratio and serum 1,25-dihydroxyvitamin D that reflects the dependence of net bone resorption on 1,25-dihydroxyvitamin D was upheld in hypoparathyroid patients. It is proposed that the subjects with familial hypoparathyroidism in this kindred had moderate asymptomatic hypocalcemia without developmental abnormalities because normal or elevated serum 1,25-dihydroxyvitamin D levels enhanced intestinal calcium absorption. This may represent one point in the spectrum of idiopathic hypoparathyroidism. Alternately, both the moderate degree of hypocalcemia and the normal serum calcitriol values could have been related to mild, partial hypoparathyroidism, which could have been inherited in this kindred.