The serum 25 ( OH ) D concentration was the primary outcome , and serum calcium , parathyroid hormone ( PTH ) , 1,25-dihydroxyvitamin D [ 1,25 ( OH ) D ] , 24,25-dihydroxyvitamin D [ 24,25 ( OH ) D ] , the ratio of 24,25 ( OH ) D to 25 ( OH ) D , vitamin D-binding protein , and free 25 ( OH ) D were exploratory outcomes .