Talk radio host Rush Limbaugh revealed he has advanced lung cancer during his show on Monday afternoon, shocking and saddening his listeners as well as those who work on his staff — and much of the radio-broadcast industry.

“I have to tell you something today that I wish I didn’t have to tell you. It’s a struggle for me because I had to inform my staff earlier today. I can’t escape. Even though people are telling me it’s not the way to look at it, I can’t help but feel that I’m letting everybody down with this,” he said.

“But the upshot is that I have been diagnosed with advanced lung cancer, diagnosis confirmed by two medical institutions back on Jan. 20. I first realized something was wrong on my birthday weekend, Jan. 12,” Mr. Limbaugh told his 14 million-member audience.

Sympathetic reactions poured in immediately from social media.

“Those of you who are listening to the Rush Limbaugh show now. Pray with us. Thank you. God Bless you Rush Limbaugh. Love you so much Rush,” tweeted James Golden — known on the air as “Bo Snerdley,” Mr. Limbaugh’s longtime producer and call-screener.

The huge response to Mr. Golden’s tweet was telling, revealing shock, dismay and sadness among listeners. The hash tag #rushlimbaugh immediately dominated national trends on Twitter.

“I’m floored,” wrote one fan. Another wrote: “Rush has been part of our family for 30 years. He, his family and medical staff are in our prayers for strength and healing.”

Michael Harrison, editor of Talkers Magazine, a talk radio industry publication, said that “anything I have to say about Rush’s importance to the radio industry as well as the national conversation has been said countless times before.

“He is cumulatively the most influential individual in the modern era of talk media and any change in his role will have enormous impact. We pray for his swift and complete recovery,” Mr. Harrison said.

Details about Mr. Limbaugh’s illness, treatment and future broadcast plans are forthcoming, but he was as candid as possible with his listeners.

“My intention is to come here every day I can and to do this program as normally and as competently and as expertly as I do each and every day, because that is the source of my greatest satisfaction professionally, personally,” he said.

“I’ve had so much support from family and friends during this that it’s just been tremendous. I told the staff today that I have a deeply personal relationship with God that I do not proselytize about,” Mr. Limbaugh said.

“We’re at full-speed ahead on this,” he added.