The sister of an emergency room doctor who died by suicide while helping fight the coronavirus pandemic said that before her death, Dr. Lorna Breen had been in an “untenable” situation.

Speaking to “TODAY’s” Savannah Guthrie, Jennifer Feist said her sister was overcome by a grim combination of events. She contracted the virus, which Feist believed “altered her brain.” Eventually, Breen returned to work.

“She went back to the most horrific, unimaginable conditions,” Feist said, describing her sister as “somebody whose life’s calling is helping people, and she just couldn’t help enough people.”

“And the combination was just untenable,” Feist said.

Watch the full interview Thursday on “TODAY” at 7 a.m.

Breen, 49, had worked at NewYork-Presbyterian Allen Hospital in northern Manhattan. Her father told The New York Times that after she got sick and returning to work a week and a half later, her family intervened and brought her to Charlottesville, Virginia.

Authorities said Monday that she was taken to UVA University Hospital, where she succumbed to self-inflicted injuries.

Before her death, Feist said, Breen had been working 12-hour shifts.

“When she finished, she said: ‘I can’t leave. Nobody’s leaving. I have to stay and help,'” Feist said. “And I kept telling my sister: ‘You know, you can’t — if you can’t function, you can’t help anybody. You have to sleep. You have to rest.'”

Feist added that Breen didn’t want to give up.

“She would not let it break her, which, of course, it did,” she said.

The hospital didn’t immediately respond to a request for comment, but in a statement Monday it called Breen “a hero who brought the highest ideals of medicine to the challenging front lines of the emergency department.”

If you or someone you know is in crisis, call the National Suicide Prevention Lifeline at 800-273-8255, text HOME to 741741 or visit SpeakingOfSuicide.com/resources for additional resources