Ken Rosenthal of The Athletic first reported the failed test on Wednesday.
On Thursday, Castillo issued a statement, revealing he tested positive for erythropoietin (EPO) and that he took "full responsibility" for the positive test, via Yahoo Sports MLB:
Castillo had been the team's primary catcher this season, hitting .270 with six home runs and 15 RBI in 32 games.
The 31-year-old has been especially hot as of late, hitting .319 in 13 games in May. He also came through with a season-high three hits in Tuesday's win over the Baltimore Orioles.
Castillo was in the lineup Wednesday, but it appears to be his last game for a few months.
The Dominican Republic native is in his first season with the White Sox after signing a two-year deal in the offseason worth over $15 million. He was coming off the best year of his career, setting career highs with the Orioles in home runs (20), batting average (.282) and OPS (.813) in a full season.
Unfortunately, the latest news is another disappointing development for a team that holds a 15-31 record, third-worst in the majors.
Omar Narvaez figures to get the majority of playing time behind the plate with Castillo unavailable, although he has struggled offensively this season with a .180 batting average in 61 at-bats.