(CNN) -- Former "Good Morning America" co-host Joan Lunden revealed on Tuesday that she has been diagnosed with breast cancer.
Posting to fans via Twitter and a letter on her website JoanLunden.com, Lunden said she had received an all-clear two weeks ago after her annual mammogram, but "for women who have dense fibrous breast tissue, as I do, often our doctors will recommend an ultrasound as well."
"My ultrasound that day revealed a tumor in my right breast," she wrote. "After a core biopsy was performed, I heard those words that every woman fears and that I never thought I'd hear: 'you have breast cancer.' "
Lunden, who co-hosted "GMA" from 1980 to 1997, said she initially considered keeping the news private, but decided to go public with her diagnosis.
"I have decided to talk about my breast cancer because since the moment I took the job at 'Good Morning America' I have lived my life sharing my joys and my disappointments with all of you: my pregnancies, my relationships, my career," she said. "I have shared my journey. So it certainly didn't feel right keeping this part of my journey a secret."
Lunden turned to current "GMA" co-host and breast cancer survivor Robin Roberts to offer her first TV interview about having the disease. The news hit home for the staff, as "GMA" co-host Amy Robach is currently being treated for breast cancer.
Lunden told Roberts that the form of cancer she has is particularly aggressive.
"In the beginning it's such a shock, it's a stunner," Lunden told Roberts. "For me, I'm a health advocate, I'm all over America talking to women saying, 'Make sure you get all of your checkups.' In the beginning you almost feel like 'What did I do wrong?'"
Lunden, whose late father was a cancer surgeon, said she gets mammograms every year. She is now in "warrior mode" learning everything she can about the disease and is currently undergoing chemotherapy, she said.