Breaking news: Mike Gastineau has tested positive for sanity. After 21 years, he’s leaving KJR-AM sports radio for  . . .  well, life.

“It been an unbelievable run,” he said Tuesday after giving notice. “It got to a point where I’d like to try something different. I’m just not sure what that is. But I’m not going somewhere else and not coming back.”

Gastineau, 52, said the parting was amicable — no firing or disputes — and that he has no other radio job offers.

“I can’t thank Mike enough – both personally and professionally, for his talent, dedication and passion,” Rich Moore, KJR’s program director, said in a statement. “Mike has been a pillar for KJR and a sports icon in the Seattle market. His voice in the afternoon will be missed and his relationship with listeners in the market will not be forgotten. We wish him nothing but the best in his next chapter.”

Gastineau, whose last show with co-host Elise Woodward is Thursday, has done more than 5,000 shows since his Seattle arrival in 1991 with, as he described it, “my last $900 and all my possessions in a Dodge Daytona hatchback.”

KJR became the market’s first all-sports-talk format in September 1991. By September 1993, Gastineau had taken over the afternoon drive slot from 3-7 p.m. and has been one of the most popular, respected and longest-tenured figures in the Seattle sports-media scene.

As much for his quick wit and rational thinking, Gastineau is known as a prolific fundraiser for charity. The driver behind the station’s annual “KJR Kares-a-thon,” he has raised more than $1.5 million over 17 years for various Puget Sound-area charities.

Gastineau made his radio mark in hosting public discussions around Seattle’s sports-facilities developments, from KeyArena to Emerald Downs to the baseball and football stadiums, and lately the campaign to create a new basketball/hockey arena in SoDo.

“I’m proud to have been able to give Seattle fans a voice in these incredibly important debates,” he said. “I always strived to do three things on the show – be funny, entertaining and give people a place for reasonable debate on sports issues of the day.”

Along with this writer and Sportspress Northwest co-founder Steve Rudman, Gastineau in 2009 co-authored “The Great Book of Seattle Sports Lists.”

When Gastineau, an Indiana native, made it to Seattle, KJR was owned by the Ackerley family, which also owned the NBA Sonics, and KJR became the NBA flagship station in the market. The Ackerleys sold the Sonics in 2000 to a group led by Howard Schultz and its radio stations in 2001 to ClearChannel of San Antonio, which itself was purchased in 2008 by Bain Capital the same month the Sonics moved to Oklahoma City.

ClearChannel, the nation’s largest radio chain with 850 stations, including six others in the Seattle market, was purchased by Bain and another private equity firm, T.H. Lee, for $26.7 billion. According to ClearChannel’s Nov. 2 filing with the U.S. Securities and Exchange Commission, the company had $16.4 billion in debt.

Competition grew tighter in the Seattle radio market in April 2009 when KIRO-AM switched from to news-talk to sports-talk, taking on ESPN programming around nine hours of local programming. ESPN 710 recently had its contract renewed to carry the Seahawks, and remains the Mariners flagship station, and this year picked up Washington State University football.

KJR has the rights to University of Washington football and basketball. A third sports-format station, backed by CBS Sports, will begin in January.

Gastineau made it clear he wasn’t retiring.

“The explosion around the Internet and sports has been amazing,” he said. “I have a few creative ideas I’d like to explore.”