michael f. holick (born 1946) is an american adult endocrinologist, specializing in the field of vitamin d, such as the identification of both calcidiol, the major circulating form of vitamin d, and calcitriol, the active form of vitamin d. his work has been the basis for diagnostic tests and therapies for vitamin d-related diseases. he is a professor of medicine at the boston university medical center and editor-in-chief of the journal "clinical laboratory".