The 32-year-old was sent off after colliding with Mohamed Diame in the box during Wednesday night's 2-1 defeat at Upton Park.
It has been revealed by manager Steve Bruce today that McGregor is likely to miss the rest of the season after he suffered kidney damage as a result of the clash.
With regards to Hull's successful appeal, an FA statement said: "Hull City's Allan McGregor has had his claim for wrongful dismissal upheld following an Independent Regulatory Commission hearing today.
"McGregor was dismissed for denying a goal scoring opportunity during the game against West Ham United on March 26, 2014. The player's standard punishment, therefore, has been withdrawn with immediate effect."
The shot-stopper was taken to hospital after he left the field.