(Reuters) - A top U.S. healthcare official on Thursday challenged claims by Republican lawmakers that the country's healthcare reform law is causing workers to lose hours or benefits, saying she is aware of only isolated cutbacks. At a stormy House of Representatives committee hearing, Marilyn Tavenner, who heads the Medicare and Medicaid programs, said she has found only anecdotal evidence of employers reducing work hours or benefits because of worries about President Barack Obama's landmark reforms, which take effect on January 1.