I'm working at NIH and many of the MD-fellows here (I know very specifically only of pediatric endocrinologists) have chosen to do quite involved basic science projects. It might be a good idea to find an institution where you'll have the option to do good basic science during your fellowship, then find the lab with perhaps an MD/PhD PI who will be willing to give you a good bit of mentoring. If you've enjoyed your basic science training, then there's always the option option of doing a post-doc afterward. And if you haven't, well, you're ready to practice medicine.