Sykes, 70, of Seattle, had been missing for three days when a search for her was suspended Saturday afternoon after the discovery of a body.
The cause of her death, which was ruled an accident, was hypothermia, the Pierce County Medical Examiner's Office told the Los Angeles Times. Coroner's officials said arteriosclerotic vascular disease, a type of hardening of the arteries, was a contributing factor.
Her boyfriend and hiking partner, Robert Morthorst, declined to comment.
"Ms. Sykes is a very strong hiker and she wanted to go ahead, and her hiking partner had chosen not to hike on the snow, so they agreed to meet each other back at the same point," said spokeswoman Mimi Gorman, when crews were still searching.
Morthorst reported Sykes missing about 10:45 p.m. Wednesday. Park officials later said she was carrying enough survival gear for one night.
Her death comes less than a month after six climbers trying to reach Mt. Rainier's summit disappeared in late May. All are presumed dead. Park officials suspended efforts to recover their bodies in the treacherous terrain.