Interrelationship of serum 25-hydroxyvitamin D3 and 1,25-dihydroxyvitamin D in juvenile renal osteodystrophy after therapy with 25-hydroxyvitamin D3.

The interrelationship of circulating 25(OH)D3 and 1,25(OH)2D in children with uremic renalosteodystrophy has been examined in 19 patients treated for 1 to 4 yr with 25(OH)D3. There was a direct correlation between 25(OH)D3 and 1,25(OH)2D levels when the GFR was less than or equal to 25 but not greater than 25 ml/min/1.73m2 (r = 0.47, P = less than .01). This correlation was unchanged when 1,25(OH)2D was factored by serum phosphorus or serum creatinine but became nonsignificant when 1,25(OH)2D was factored by serum bicarbonate. Overall, parathyroid hormone (PTH) did not change the correlation. However, when the data were subdivided by levels of glomerular filtration rate (GFR) and PTH, the correlation between 25(OH)D3 and 1,25(OH)2D was significant only when GFR was less than or equal to 25 and the PTH was markedly elevated (P = less than .01), suggesting that a substantial elevation of PTH may be necessary to stimulate 1,25(OH)2D production in these patients. Individual 1,25(OH)2D levels measured over time (1-4 yr) fluctuated regardless of GFR. Despite high serum 25(OH)D3 levels after 25(OH)D3 therapy, there was no consistent increase in the 1,25(OH)2D levels, and many posttreatment levels were lower than pretreatment levels. The kinetics of 1,25(OH)2D metabolism failed to reveal a progressive increase in the serum levels between 2 and 24 hours after a pharmacologic dose of 25(OH)D3.