Former talk show host Oprah Winfrey is leaving WeightWatchers board of directors and donating all of her interest in the company to a museum.

Shares of WW International Inc. tumbled more than 23% in Thursday morning trading.

Winfrey, who told People magazine in December that she was using a weight-loss medication, has served on the company's board since 2015. She will not stand for re-election at WeightWatchers annual meeting in May.

WW International said in a regulatory filing that Winfrey's decision "was not the result of any disagreement with the company on any matter relating to the company's operations, policies or practices." The size of its board will go from 10 to nine members following its annual meeting, the New York company added.

"I look forward to continuing to advise and collaborate with WeightWatchers and CEO Sima Sistani in elevating the conversation around recognizing obesity as a chronic condition, working to reduce stigma, and advocating for health equity," Winfrey said.

According to FactSet, Winfrey's stake of about 1.1 million shares made her the company's largest individual shareholder, with a stake of 1.43%.

Winfrey said that she will donate her interest in WeightWatchers to the National Museum of African American History and Culture, part of the Smithsonian Institution in Washington. The company said that it supports Winfrey's decision to donate all of her stake to the museum during WeightWatchers upcoming trading window in March.

"Ms. Winfrey is making the donation to support the NMAAHC's goal to promote and highlight the contributions of African Americans and to eliminate any perceived conflict of interest around her taking weight loss medications," the company said. "In addition, Ms. Winfrey intends to donate the proceeds from any future exercises of her WW stock options to NMAAHC."

Nearly a year ago, WeightWatchers said it too was getting into the prescription drug weight loss business with a $106 million deal to buy Sequence, a telehealth provider with annual revenue of about $25 million and about 24,000 members.