The Chinese actor known for his kung fu skills discussed his diagnosis during the taping of a talent show he's judging in China.
During Tuesday's taping, the 50-year-old said his weight has fluctuated due to the condition, but he's taking it all in stride. Li joked about his weight gain and said "I'm fat, I don't have the time to lose it. It's a fact!"
Li explained that exercise is not advised with the medicine he's taking. An overactive thyroid, or hyperthyroidism, causes changes in a person's metabolism and heart rate, among other symptoms, but is generally treatable with medications.
Li was diagnosed in 2010. He kept his condition under control with medication, but it came back with a vengeance recently.
"I'm just a regular guy, I'm not Wong Fei Hung, I'm not Huo Yuan Jia (kung fu heroes he portrays on film), I'm not a hero. I'm just like you," he said.
Li was an accomplished martial arts champ at a young age. He turned to acting and began showing off his kung fu skills on big screens in the 1980s.
He rose to fame in the '90s with the "Once Upon a Time in China" films where he portrayed martial arts master Wong Fei Hung.
His Hollywood career includes such titles as "Lethal Weapon 4," ''Romeo Must Die" and more recently "The Expendables 2."
Li confessed there are times he's unsure if he's able to carry on with work, but he's determined. "I'm in pain, but I'm not suffering. I'm happy," he said.
The Associated Press contributed to this report.