<prompt> Which of the following is the major circulating form of Vitamin D?25-OH D31, 25-OH2 D324, 25-OH2 D31, 24-OH2 D3  <response>: 25-OH cholecalciferol is the major circulating form of vitamin D because of the half-life of 3-weeks 1,25-dihydroxy vitamin D3 a.k.a. calcitriol is the biologically active form vitamin D. But, its half life is low. In clinical laboratory, we measure 25-OH cholecalciferol. 