"Good Morning America" co-anchor Robin Roberts rejoined her fellow TV show hosts on Monday after taking a three-week leave.

"Merry Monday @GMA. Great to be back with George, Sam, Josh, Lara and YOU! Rested and rarin' to go again...you have been warned. :-) xxoo," she tweeted early Monday morning.

In June, Roberts, 51, announced that she had been diagnosed with myelodysplastic syndrome (MDS), a rare blood and bone marrow disorder. Five years ago, Roberts beat breast cancer, but some survivors can suffer from other diseases which are triggered by treatments, such as chemotherapy, WebProNews reported .

During her time off, Roberts helped raise money for cancer research in California, visited friends and family in Mississippi and celebrated a friend's 50th birthday in Italy. She even received special access to the Vatican, where she prayed at the altar the pope uses before addressing the crowd from his balcony, US Weekly magazine reported.

"I was saying a special prayer for all those who have lifted me up in prayer," Roberts wrote on Twitter. During her trip, Roberts tweeted: "Grateful to Italy for renewing my spirit/soul. On way back now to good ol USA! There's no place like home. Blessings to all. Happy Sat..:-)"

The broadcast journalist let her coworkers and viewers know that she is ready to get back to work. "My needle was past 'E' when I left here a couple of weeks ago, but I've got a full tank, so watch out. I haven't taken this much time off in quite some time."

Nearly three weeks ago, Roberts abruptly left "GMA," and said: "Well a full disclosure here, I'm not feeling too well. In fact, I'm going to leave, and let you all do the rest of the program on your own. I'm going to take a little time off, just to get some vacay. I'll see you in a couple of weeks."

Roberts continued to receive treatment during her temporary leave; however, the PICC line -- a catheter used in chemotherapy -- was removed from her arm. She will undergo surgery for a bone marrow transplant from her sister in a few weeks, at which point Roberts will go on a longer medical leave.

After Monday's show, Roberts thanked her Twitter followers for a "great welcome back" August 20. "Going right now to tape a PSA for Be the Match," she wrote. "Bone marrow donors (like my sis) are saving lives."

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Good Morning America, Robin Roberts