"Jeopardy!" host Alex Trebek revealed that he is undergoing chemotherapy again after his "numbers went sky-high" shortly after he finished his first round of treatment for stage 4 pancreatic cancer.

"I was doing so well and my numbers went down to the equivalent of a normal human being who does not have pancreatic cancer. So we were all very optimistic," Trebek said in an interview that aired Tuesday on "Good Morning America," adding that because he was responding so well doctors decided to stop chemotherapy and start him on immunotherapy treatment.

"I lost about 12 pounds in a week and my numbers went sky-high, much higher than they were when I was first diagnosed. So, the doctors have decided that I must undergo chemo again. So that's what I'm doing."

Trebek, 79, first revealed his cancer diagnosis in a YouTube video in March. A few months later, in August, the longtime "Jeopardy!" host announced that he had completed chemotherapy and was back taping Season 36 of the game show, which aired its first episode on Sept. 9.

Trebek said on "Good Morning America" that chemotherapy has taken a toll on his body and sometimes causes "excruciating pain" in his lower back and leaves him fatigued and nauseous.

The popular host also said there are times he feels "this surge" of sadness and depression, but it doesn't last long.

Despite the setback in treatment, Trebek said he is "enjoying what's going on."

"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.'" he said. "Hey, guys. I’m 79-years-old. I’ve had one hell of a good life. And I’ve enjoyed it."

He added: "The thought of passing on doesn't frighten me. Other things do, the affect 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.”

Trebek, who has been the host of "Jeopardy!" since 1984, said he still plans to work as long as he is able to do so.

The Pancreatic Cancer Action Network said it is not uncommon for those with pancreatic cancer to initially respond well to treatment and then require further chemotherapy.

The organization said in a statement to NBC News that it "commends Alex Trebek for his continued openness and honesty about his treatment journey" and wishes him a "good response."