US STOCKS-Wall St ends lower as Caterpillar, health stocks weigh
* Yellen says expects rate hike later this year
* Caterpillar hits 5-year low after sales forecast cut
* Health stocks see 5th day of losses, biotech index falls 2 pct
* Investors await Yellen speech at 5 p.m. ET
* Indexes down: Dow 0.48 pct, S&P 0.34 pct; Nasdaq 0.38 pct (Updates with futures up in volatile trade after the bell following Yellen speech)
By Sinead Carew
Sept 24 (Reuters) - U.S. stocks closed lower on Thursday in a volatile session on uncertainty about U.S. monetary policy and global economic growth, while market heavyweight Caterpillar cut its sales forecast and healthcare investors fled for the exits.
After the bell, Federal Reserve Chair Janet Yellen said she expects the Fed to raise interest rates later this year. She was said to receive medical attention after her speech, though a spokesman for the University of Massachusetts at Amherst, where she spoke, said she was expected to resume her schedule.
S&P 500 e-mini futures were down at the start of the overnight session but later turned higher and were last up 0.4 percent. Continuación...