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 (25OHD) 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 malabsorption-associated bone disease. Plasma 25OHD 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.