Dreyfus took to Twitter Thursday afternoon to break the news and call for universal health care in the United States.
"1 in 8 women get breast cancer. Today, I'm the one," she wrote. "The good news is that I have the most glorious group of supportive and caring family and friends, and fantastic insurance through my union. The bad news is that not all women are so lucky, so let's fight all cancers and make universal health care a reality."
Her post prompted messages of love and well wishes from a number of fellow celebrities, including her Veep co-stars Tony Hale and Sam Richardson. "We love this woman," Hale wrote.
"She is one of the strongest and caring people I know," Richardson added. "Nothing can stop her. Not even this."
See a selection of the responses below.