Robin Williams may have committed suicide because he feared the disease may have stopped him cycling, a sport he took up to battle his drink and drugs addiction.
According to friend Tony Tom, 63, the actor and comedian may have become afraid that he would fall into the grip of his addictions again, when the degenerative nervous disorder made cycling impossible.
"It (Parkinson's) would have affected his ability to ride, no doubt," Tom told The Sun.
"He could have turned back to his earlier vices if he wasn't riding and maybe that was one of the things he was thinking about."
Williams was an avid cycling fan, and was friends with Tour De France winner Eddy Merckx and Lance Armstrong.
When asked how many bicycles he owned, he once quipped "too many to count".
It is believed that Williams' interest in the sport was inspired by Armstrong, who was stripped of seven Tour De France wins after admitting taking performance enhancing drugs in 2012.
The Mrs Doubtfire star took part in the Ride for Roses event in Texas, which was organised by the charity foundation Livestrong.
During a rest day of the 2002 tour, Williams rode out with Armstrong's US Postal team, later donating his kit to charity.
Through attending the tour he came to know Belgian cycling legend Merckx, who dominated the sport in the 1970s.
"I am shocked at Robin Williams' death," he was quoted saying on Belgian media.
Armstrong paid tribute to Williams on Twitter: "RIP Robin. I will always remember you as one hell of a friend. I love you and will miss you terribly," he tweeted.
Williams, 58, was found dead in his San Francisco home on Monday. He is believed to have hung himself.
Yesterday, the last pictures of Williams alive emerged.
He attended a reception at a gallery near his home in Fairfax, California with his wife Susan Schneider, 50.
In the pictures, the star looks gaunt but was said to be in good spirits as he chatted to artists and friends.