The decision was made after further evaluation Wednesday.
The six-year veteran out of Syracuse was not cleared to play this preseason after a routine physical last week revealed an abnormality.
The 6-foot-10 Thomas has career averages of 6.2 points and 5.1 rebounds in 347 NBA games over six seasons, all with the Wizards.
"This is a difficult situation, and we wish him the best," teammate Antawn Jamison said.
A team spokesman said there would be no further comment.
Notable deaths of 2014 [PHOTOS]
UPI Sports Results for September 19, 2014