as a graduate student, he identified the major circulating form of vitamin d, 25-hydroxyvitamin d3, which is the vitamin d metabolite that is measured by physicians worldwide to determine a patient's vitamin d status. he also identified the active form of vitamin d, 1,25-dihydroxyvitamin d3, as well as other metabolites including 24,25-dihydroxyvitamin d3, 1,24,25-trihydroxyvitamin d3 and 25,26-dihydroxyvitamin d3.