Sykes, 70, disappeared last Wednesday after she left her hiking partner to explore an area up a trail on the east side of the park. Her body was found during the weekend about 5,000 feet above sea level.
Karen Barr, an investigator with the Pierce County Medical Examiner, told NBC News the cause of death was hypothermia, or exposure to the cold when the body's core temperature drops too low.
Sykes was the author of Hidden Hikes in Western Washington, as well as newspaper columns and articles about hiking in the Pacific Northwest. She was experienced in the mountains.
A spokeswoman for the park said Sykes was working on a story at the time of her death.
Sykes' daughter, Annette Shirey, told NBC during the weekend that her mother died as she would have wished.
"She always said that if her life was going to end she would want it to end in the mountains -- she wouldn't want to be anywhere else," Shirey said.