Jim Parsons reveals why he quit The Big Bang Theory as final episode airs

"The Locomotion Reverberation"-- Pictured: Sheldon Cooper (Jim Parsons). Leonard and Wolowitz try to distract Sheldon when he slows the progress of their guidance system. Also, Penny and Amy take Bernadette out for the night, leaving Raj and Stuart to care for baby Halley, on THE BIG BANG THEORY, Thursday, Feb. 9 (8:00-8:31 PM, ET/PT), on the CBS Television Network. Dean Norris returns as Colonel Williams.

The Big Bang Theory is coming to an end on Thursday (May 16) after 12 seasons and star Jim Parsons, best known for playing acerbic Sheldon Cooper, has revealed why he chose to walk away from the hit comedy.

Parsons made the decision not to renew his contract on Big Bang Theory for a potential 13th season, prompting the announcement that the long-running show will end this year.

“It was the first time in my life of doing this show that it occurred to me that I might want to not do another contract after [Season] 12 was up,” Parsons told The Hollywood Reporter.

The final episode of The Big Bang Theory will air on Thursday on CBS in the US.

Jim Parsons on the end of The Big Bang Theory

Parsons added that once he made the decision to leave the show, which also stars Kaley Cuoco and Johnny Galecki, he was resolute.

“I don’t know if it’s because I’m an Aries or just because maybe I’m in touch with myself. Whatever it is, once I had that thought, I was like, ‘Well, that’s your answer,'” said the star, who has won four Emmy awards for playing Sheldon.

“There was no factor; there was no situation that I was like, ‘Well, I’ve had enough of that.’ It was just…when you know, you know.”

The Big Bang Theory is ending after 12 seasons. (therealjimparsons/instagram)

For his part, Big Bang Theory creator Chuck Lorre said he couldn’t imagine the show continuing without Parsons leading the cast.

“I couldn’t wrap my head around the idea of going on without the whole ensemble—and the whole ensemble is why we succeeded,” Lorre told The Hollywood Reporter.

“In pulling it apart and re-approaching it as a fraction of what it was just never felt right to me. I’ve seen other shows try and fail to take a character out of their realm and carry on. And maybe Frasier is the only thing I could think of off the top of my head where it really did work, thanks to the lightning strike of David Hyde Pierce.”

How will The Big Bang Theory end?

In the final episode of Big Bang, airing on Thursday (May 16), Sheldon and wife Amy win the Nobel Prize for their scientific work.

Parsons remains part of The Big Bang Theory universe as producer and narrator of its spin-off show, Young Sheldon.