Oprah Winfrey is going public about the thyroid condition that slowed down her metabolism and caused her to gain 20 pounds.

Winfrey, 53, was feeling incredibly sluggish by the time her talk show wrapped up its season in May. She eventually discovered the problem: an out-of-balance thyroid.

“My body was turning on me,” she says in the October issue of O, the Oprah magazine. “First hyperthyroidism, which sped up my metabolism and left me unable to sleep for days. (Most people lose ...