The CBS sitcom came to an end after 12 seasons when the actor, who played Sheldon Cooper, said he was leaving.
Production designer John Shaffner has since revealed that the show was set to run for another two seasons before Parson announced his decision.
Now, in an interview on David Tennant's podcast, the actor described the process as "a complicated road".
"In retrospect, it was just happening before my eyes," he said. "Our final contract was for the last two years, but no one knew when we signed it what that would mean. I kind of had a suspicion in my heart that was going to be it for me when I did sign that contract. You never say never, who knows?"
It was after an "intense" summer, in which his dog was approaching the end of his life while he starred in Broadway play Boys in the Band, that the actor began to think about his future plans.
Parsons said he was left "beaten down" by the experience and started to think about the death of his father, who died when he was 52 – six years younger than Parsons's current age.
"I realised that at the end of season 12, I would be 46," Parsons continued. "I’m not superstitious, or anything like that, but it was just a context thing. If you told me that, like my father, I had six years left to live, I think there’s other things I need to try and do. I don’t even know what they are, but I can tell that I need to try."
He also said a concern with quitting the series was that the cast and crew members would all be out of a job.
"There’s always that part of it, that other people who are like I could use the work, dude.' That’s just the way this business is, and that part’s hard."