It was only last week that Alex Trebek returned to the "Jeopardy!" stage to kick off season 36 of the iconic answer and question show. The host had undergone chemotherapy for stage 4 pancreatic cancer, and announced less than a month ago that he had finished treatment and was back at work.

On Tuesday, however, Trebek revealed he was returning to chemotherapy treatment. In an interview with "Good Morning America," Trebek said his doctors want him to undergo another round of chemo because he lost a lot of weight and his numbers "went sky high."

"I was doing so well. And my numbers went down to the equivalent of a normal human being who does not have pancreatic cancer," Trebek said in the interview. "So we were all very optimistic. And they said, 'Good, we're gonna stop chemo, we'll start you on immunotherapy.'"

Alex Trebek says he lost 12 pounds in one week, causing concern and his doctor's decision for him to do one more round of chemo. "Jeopardy!"

The host revealed he lost about 12 pounds in a week and his number went much higher than they did when he was first diagnosed. "So, the doctors have decided that I have to undergo chemo again and that's what I'm doing," he said. "Cancer is mysterious in more ways than one."

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Trebek also opened up about the "excruciating pain" and "fatigue" his cancer has caused. He also said his cancer battle caused depression and a "surge of sadness."

"When it happened early on, I was down on myself," Trebek said. "I didn't realize how fallible each of us is in his or her own way … I talk to the audience sometimes and I get teary-eyed for no reason. I don't even bother to explain it anymore, I just experience it. I know it's a part of who I am and I just keep going."

Trebek, 79, has been the host of "Jeopardy!" since it was revived in 1984. His contract keeps him in the hosting position until 2022, which he joked about when announcing his cancer diagnosis. "I have to host 'Jeopardy!' for three more years! So help me. Keep the faith and we'll win. We'll get it done," he said.

Trebek has hosted "Jeopardy!" since 1984 and has no plans of stopping. "Jeopardy!"

While speaking with "GMA," Trebek said death does not frighten him. "I realize that there is an end in sight for me, just as there is for everyone else," Trebek said. "One line that I have used with our staff in recent weeks and months is that when I do pass on, one thing they will not say at my funeral is, 'Oh, he was taken from us too soon.'"

"Hey guys. I'm 79 years old. I've had one hell of a good life. And I've enjoyed it. … The thought of passing on doesn't frighten me, it doesn't. Other things do, the effect it will have on my loved ones ... it makes me sad. But the thought of myself moving on, hey folks, it comes with the territory," the host said.

Still, Trebek says he has not thought about leaving "Jeopardy!" just yet. "As long as I can walk out and greet the audience and the contestants and run the game, I'm happy," he said.