Vitamin D4 is the active analogue of Vitamin D.
[1]. Byford V, et al. Use of vitamin D(4) analogs to investigate differences in hepatic and target cell metabolism of vitamins D(2) and D(3). Biochim Biophys Acta. 2002 Jul 11;1583(2):151-66. [2]. Tachibana Y, Tsuji M. Study on the metabolites of 1alpha,25-dihydroxyvitamin D4. Steroids. 2001 Feb;66(2):93-97. [3]. Tsugawa N, et al. Biological activity profiles of 1alpha,25-dihydroxyvitamin D2, D3, D4, D7, and 24-epi-1alpha,25-dihydroxyvitamin D2. Biol Pharm Bull. 1999 Apr;22(4):371-7. [4]. Rikkers H, DeLuca H, Lobeck C. Metabolism and serum protein binding of 3H-vitamin D4 in vitamin D-resistant rickets. J Pediatr. 1969 May;74(5):828. [5]. De Luca HF, Weller M, Blunt JW, Neville PF. Synthesis, biological activity., and metabolism of 22,23-3H vitamin D4. Arch Biochem Biophys. 1968 Mar 20;124(1):122-8.