4:15 PM PT -- Officials believe Jeremy Giambi died by suicide, law enforcement sources tell us.
We're told an emergency call was received at 11:40 AM Wednesday morning requesting medical assistance at the Giambi household. When they arrived, we're told first responders found Jeremy deceased.
Ex-MLB player Jeremy Giambi -- who played in the MLB from 1998 to 2003 -- has passed away, TMZ Sports has learned. He was 47 years old.
Giambi played for the Royals, Athletics, Phillies, and Red Sox over the course of his career ... earning a .263 batting average and hitting 52 home runs and 209 runs batted in.
His best season came in 2002, when he played in 124 games and hit 20 homers.
The A's issued a statement on Jeremy's death, saying ... "We are heartbroken to learn of the passing of a member of our Green and Gold family, Jeremy Giambi. We offer our condolences to Jeanne, Jason, and his family and friends."
Jeremy is the brother of Jason-- the American League MVP in 2000, who played for the Athletics, Yankees, Rockies and Indians.
Giambi was portrayed in the 2011 Brad Pitt flick "Moneyball" ... and was played by actor Nick Porrazzo. His character was depicted as a partier and disruption during his time with the A's ... and a scene shows him eventually being traded to the Phillies.
Giambi was part of the most memorable baseball moments of the past few decades ... when Derek Jetermade a spectacular play to throw him out at home plate in the 2001 American League Divisional Series.