The present invention provides a method for treating diseases caused by either an excess or diminution of Vitamin D₃. It provides pharmaceutical compositions for the treatment of such diseases and the methods by which these compositions are to be used. Also provided are methods for testing the activity of both 24(OH)ase and CCAATT/Enhancer Binding Protein (C/EBP), as well as, for testing the effect of a compound on 24(OH)ase and C/EBPB activity activity. In particular embodiments, a method is provided for reducing the risk of hypercalcemia when administering 1,25(OH)₂D₃ or its analogs for the treatment of vitamin D diseases. Methods for both enhancing and diminishing 24(OH)ase activity are also provided.