Roberts announced her tweet was regarding "something I have been dealing [with] for the last couple months" on ABC's "Good Morning America" Monday morning.

"Today, I want to let you know that I've been diagnosed with MDS or myelodysplastic syndrome. It's a disease of the blood and bone marrow and was once known as preleukemia," Roberts wrote on ABC's blog. "My doctors tell me I'm going to beat this — and I know it's true."

Obama's Twitter feed directed followers to Roberts's post in order to understand the diagnosis. Roberts is also encouraging readers to sign up to be bone marrow donors.

Roberts has interviewed the first lady in the past, as recently as late last month, when Obama was promoting her book, American Grown.