After finishing her 21st season of The Oprah Winfrey Show, Oprah says she had a serious wake-up call about her health—one that every woman over 35 needs to pay attention to.

"At the end of May, I was so exhausted I couldn't figure out what was going on in my life. I ended up going to Africa and spent a month with my beautiful daughters there, was still feeling really tired, really tired, going around from doctor to doctor trying to figure out what was wrong and finally figured out that I had literally sort of blew out my thyroid," Oprah says.

After her diagnosis, Oprah says she turned to a book she keeps next to her bed—Dr. Christiane Northrup's The Wisdom of Menopause. "I pulled out that book and realized that I was not alone. That 25 percent of perimenopausal and menopausal women experience some kind of issue with their thyroid at some time and most women don't know that that's what it is."

That's why Oprah says she decided to share her story about her thyroid in the October 2007 issue of O, the Oprah Magazine. "I wanted so many other women who are going through the same thing to check on yourself and recognize that, first of all, it's an issue that we all share in common, especially if you're of a certain age," she says. "It was hugely, hugely, hugely important for me to get that in balance."