Hillary Rodham Clintons campaign released a letter on Friday from her doctor attesting to Mrs. Clintons good health and fitness to serve as president based on a full medical evaluation. The letter from Dr. Lisa Bardack of Mount Kisco, N.Y., summarized Mrs. Clintons history of treatment for a brain concussion, blood clots affecting her legs and brain on separate occasions, an underactive thyroid gland and a family history of heart disease.