Dr. Susan Desmond-Hellmann is stepping down as CEO of the Bill & Melinda Gates Foundation after more than five years in the role, the foundation announced Thursday.

Desmond-Hellman, 62, was the third CEO in the organization’s 20-year history — and with a background as a research scientist and physician, the first to come from outside Microsoft. She cited family and health as reasons for leaving the world’s richest philanthropy, calling it “the toughest decision” of her career.

“But one of my mantras is ‘take your own pulse first,'” Desmond-Hellmann said in a tweet. “Over the last few months, I’ve done just that and concluded that I need to slow down.”

The foundation’s president of global policy and advocacy and chief strategy officer, Mark Suzman, will become CEO in February.

Suzman has been with the foundation since 2007, and before that held multiple positions at the United Nations, including senior adviser for policy and strategic communications for the office of Secretary-General Kofi Annan.

During Desmond-Hellmann’s tenure as the foundation’s CEO, she oversaw its first major anti-poverty investment in the United States, the establishment of the Gates Medical Research Institute, increased funding to improve access to birth control and support women in developing countries, and a record-breaking grant to the University of Washington for health research.

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“Sue brought an incredible set of attributes to the foundation: scientific expertise, tested leadership skills, a passion for building a strong internal culture, and, above all, a dedication to the mission of making the world a healthier, more equal place,” founder Melinda Gates said in a statement.

Desmond-Hellman took the helm in 2014 at a pivotal time for the foundation, as it had nearly doubled in size under her predecessor and underwent a major reorganization. At the time, founders Bill and Melinda Gates said they sought her out for her experience in public-health policy, research and development, and higher education.

She left the University of California, San Francisco, where she had been serving as chancellor, for the job. Prior to that, Desmond-Hellman oversaw drug development at Genentech, including the development of the first gene-targeted breast cancer drug. In the ’80s and ’90s, the oncologist treated and researched AIDS-related cancer in San Francisco and Uganda.

Seattle Times reporter Sandi Doughton contributed to this report.