By Ariane de Vogue and Veronica Stracqualursi, CNN

(CNN) -- Retired Justice Sandra Day O'Connor revealed in a letter on Tuesday that she has been diagnosed with the "beginning stages of dementia, probably Alzheimer's disease."

"I will continue living in Phoenix, Arizona surrounded by dear friends and family," she wrote and added, "While the final chapter of my life with dementia may be trying, nothing has diminished my gratitude and deep appreciation for the countless blessings of my life."

Chief Justice John Roberts praised O'Connor in a statement Tuesday as a "towering figure" and a "role model not only for girls and women, but for all those committed to equal justice under law."

O'Connor, 88, was nominated to the bench by President Ronald Reagan as the first female Supreme Court justice of the United States in 1981. She retired from the bench in 2006, in part to care for her husband, who was ailing from Alzheimer's.

In her retirement, she became an advocate for Alzheimer's disease as well as launching iCivics, a website dedicated to encouraging young people to learn civics.

In her letter, O'Connor also announced that she will be stepping away from public life and her leadership role with iCivics in light of her physical condition.

"It is time for new leaders to make civic learning and civic engagement a reality for all," she wrote, adding, "I hope that I have inspired young people about civic engagement and helped pave the pathway for women who may have faced obstacles pursuing their careers.

The letter was released by the court's Public Information Officer. O'Connor signed it at the bottom writing "God Bless you all."

Roberts said that while he was "saddened to learn" of O'Connor's diagnosis, he "was not at all surprised that she used the occasion of sharing that fact to think of our country first."

"Although she has announced that she is withdrawing from public life, no illness or condition can take away the inspiration she provides for those who will follow the many paths she has blazed," Roberts wrote.

O'Connor inspired generations of female lawyers who admired her path-marking success in a field that had been dominated by men. Over time, on the court she was known as a moderate conservative and often the swing vote on hot-button social issues.

"Justice Sandra Day O'Connor is a trailblazer in every sense. It's a testament to her patriotism that she is approaching this difficult new chapter (with) the same selflessness that defined her legal career," House Minority Leader Nancy Pelosi, the first female speaker of the House, posted on Twitter. "I pray that she continues to be blessed (with) her trademark strength and grace."

Sen. Susan Collins, a Maine Republican who co-founded the Congressional Task Force on Alzheimer's Disease, said on Twitter that she wasn't surprised O'Connor was facing her diagnosis with "the strength (and) bravery that have defined her life." California Democratic Sen. Kamala Harris tweeted that O'Connor "paved the way" and showed young girls they could also become a Supreme Court justice.

"In all aspects of her life Justice O'Connor has been a transparent leader, and being forthcoming about her diagnosis is another demonstration of this," the Alzheimer's Association said in a statement. "We commend Justice O'Connor for bravely sharing her diagnosis and increasing awareness about this devastating disease."