Direct effect of calcitriol on the regulation of parathyroid hormone secretion in a case of pseudo-hypoparathyroidism (a 24-month follow-up study).

The diagnosis of pseudohypoparathyroidism with osteitis fibrosa was made in a 51-year-old woman on the basis of hypocalcaemia, elevated plasma PTH (1-84) and blunted cAMP response to hPTH infusion. Radiologically, widespread signs of hyperparathyroidism were observed and quantitative histomorphometry confirmed the increased bone cellular turnover. Treatment with calcitriol (1,25 dihydroxyvitamin D3) induced a dramatic improvement of bone lesions. During treatment PTH (1-84) normalized with high dosage of calcitriol in spite of low or subnormal levels of serum calcium, and subsequently increased for each reduction of calcitriol dosage despite normal calcium levels. Our observations support a major and direct effect of 1,25 dihydroxyvitamin D3 on the regulation of parathyroid secretion of parathyroids glands.