The Effects of Isotretinoin on the Axial Skeleton and the Retinoid Effect

To the Editor.  —I read with great interest the articles by Tangrea et al¹ and Milstone et al² in the July 1992 issue of the Archives. These articles addressed the effects of isotretinoin on the axial skeleton and demonstrated the confusing nature of the retinoid effect. Tangrea et al¹ noted the occurrence of hyperostosis, while Milstone et al² discussed osteoporosis. What confuses me is how vitamin A derivatives are able to mediate both the formation of new areas of calcification in the body and to decrease the amount of calcium perceived to be in bone without having a consistent effect on serum parathyroid hormone, calcium, 25-hydroxyvitamin D, or 1,25-dihydroxyvitamin D. It is certainly possible that retinoids may induce collagen-producing cells to produce bone, inhibit osteoblastic function, and/or stimulate osteoclastic function, and that this effect is too small to significantly influence the body's hemostatic mechanism. However, since