Privational and malabsorption metabolic bone disease: Plasma vitamin D metabolite concentrations and their relationship to quantitative bone histology

Plasma 1,25-dihydroxyvitamin D₃ [1,25(OH)₂D₃] and 25-hydroxyvitamin D (250HD) concentrations were measured in twenty patients with metabolic bone disease due either to privational causes (10 patients) or malabsorption (10 patients). Abnormally low plasma 1,25(OH)₂D₃ levels were found in eleven patients, six with privational and five with malabsorption bone disease. Normal plasma 1,25(OH)₂D₃ concentrations were found in the remaining nine patients; of these, five were either receiving anticonvulsant therapy or had been hospitalised prior to investigation. In the absence of either of these factors, normal plasma 1,25(OH)₂D₃ levels were only found in patients with malabsorptionassociated bone disease. Plasma 250HD levels were below normal in eleven patients; six had malabsorption and five had privational bone disease. In the fifteen patients not receiving anticonvulsants there were significant inverse correlations between plasma 1,25(OH)₂D₃ levels and the osteoid volume, surface and seam thickness index. This study indicates that plasma 1,25(OH)₂D₃ concentrations are low in privational osteomalacia in the absence of anticonvulsant therapy or hospitalisation, although normal levels may occur in malabsorption metabolic bone disease uncomplicated by these factors. The plasma 1,25(OH)₂D₃ concentration appears to be inversely related to the histological severity of bone disease in patients not receiving anticonvulsant therapy.