Updated at 5:15 p.m.

The powerful leader of the Chicago Teachers Union is retiring, stepping aside after re-energizing the labor union and leading it through the first teachers strike in 25 years.

Karen Lewis, 64, has been battling brain cancer since 2014 and underwent surgery earlier this month. Lewis said it was time to focus on her health.

“There are a lot of people that are capable of carrying on the excellent work because the real work happens in each school building,” Lewis told WBEZ on Friday, “with each [CTU] delegate making contact with the people in their buildings … Not at headquarters.”

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Lewis said CTU Vice President Jesse Sharkey will take over, per the union constitution. As Lewis’ illness has sidelined her in recent years, Sharkey already has been standing in as the union’s public face.

The next CTU leadership election is scheduled for the spring. Lewis’ slate, the Caucus Of Rank-and-file Educators (CORE), which was elected together as a group, is expected to face a spirited challenge.

Asked what Lewis wants for Chicago schools going forward, she was ready with her famously quick wit.

“A new mayor, a new governor,” Lewis said. “That’s what I’d like to see politically — some real changes.” She said she also wants a city leader who understands “the need for an elected representative school board.”

Lewis considered taking on Mayor Rahm Emanuel four years ago but changed her plans after she got sick. She and Emanuel tangled in a very public way in the early days of his mayoralty — years that included the 2012 teachers strike and the historic closing of 50 schools in 2013.

But Emanuel was among the first to praise Lewis when news of her retirement broke Friday.

“I’ve seen firsthand the tenacity and drive that makes Karen Lewis a worthy advocate for Chicago children and teachers, and we’ve grown to admire each other as friends,” Emanuel said in a statement. “Karen may be stepping down from her position at CTU, but I know she’ll never stop fighting for Chicago’s children.”

Accolades for Lewis — a firebrand known for her passion and sharp tongue — rolled in Friday afternoon.

“Karen Lewis has been a tireless advocate for Chicago’s educators and students, and she’s someone I’ve been honored to call a partner and a friend for many years,” CPS CEO Janice Jackson said in a statement.  “I want to thank Karen for her commitment to promoting quality instruction and I know she will remain an important voice for our schools going forward.”

Bob Bruno, a labor relations professor at the University of Illinois, wrote a book about Lewis and the 2012 teachers strike called A Fight for the Soul of Public Education: The Chicago Teachers Strike.

“She made everybody feel strong,” Bruno said. “She made people who had felt powerless feel powerful, felt like they could make a difference. And she did it without being imperial, without being dictatorial. She was always of the membership.”

Lewis was first elected union leader in 2010 and over three terms as president led the union to new heights of influence and power. Lewis and her leadership team set their sights well beyond bread-and-butter union issues: focusing on democratizing the union and fighting for social justice, equitable, and well-resourced schools and against privatizing public education.

For Lewis, some of her proudest moments were more internal.

“When I took the job, we were very factionalized, we had five different groups running for leadership,” Lewis said. “And we were able to pull all that talent together and get some things done as a union as opposed to a lot of different divisions.”

Lewis was a longtime CPS high school chemistry teacher before being elected union president. A product of the Chicago Public Schools, Lewis was the only African-American woman in the class of 1974 at Dartmouth College. She comes from a family of CPS teachers, including her father, mother, and husband.

Follow WBEZ Education on Twitter at @WBEZeducation.