Secondary hyperparathyroidism with 1,25-dihydroxyvitamin D deficiency and pseudohypoparathyroidism in childhood: relationship between plasma 1,25-dihydroxyvitamin D and parathyroid hormone levels and urinary cyclic AMP response to exogenous PTH.

In order to clarify the complex interrelationship between serum calcium, 1,25-dihydroxyvitamin D (1,25(OH)2D), and parathyroid hormone (PTH), and the urinary excretion of cyclic AMP (cAMP) in response to exogenous PTH in pseudohypoparathyroidism (PHP) and related diseases, we investigated 3 patients with parathyroid disorders before and after treatment with 1 alpha-hydroxyvitamin D3 (1 alpha-OH-D3). Low plasma 1,25(OH)2D before treatment increased after giving 1 alpha-OH-D3 (0.1 micrograms/kg/day), whereas high plasma PTH measured by the c-terminal assay (C-PTH) decreased in all 3. No response in urinary cAMP was found before or after treatment in 2 patients with PHP type I, despite the fall of plasma C-PTH. However, in one patient with extremely high plasma C-PTH but normal N-PTH (measured by a homologous radioimmunoassay using 1-34 human PTH), urinary cAMP response to exogenous PTH was increased after treatment with 1 alpha-OH-D3. We suggest that he had pseudopseudohypoparathyroidism (PPHP) with Albright's hereditary osteodystrophy and a partial deficiency of renal 1 alpha-hydroxylase. In this patient secondary hyperparathyroidism is thought to be due to 1,25(OH)2D deficiency, and the decreased responsiveness to exogenous PTH before treatment due to excess PTH occupying renal receptors.