Tristan Thompson's Mom Andrea Thompson Dead

A source tells ET that the NBA star's ex, Khloe Kardashian, is by his side amid the tragedy.

Tristan Thompson is mourning a major loss. The 31-year-old NBA player's mom, Andrea Thompson, has died, a source tells ET. ET has reached out to Tristan's rep for comment.

"Khloe and Tristan traveled to Toronto because Tristan's mother passed away," the source says of Khloe Kardashian, Tristan's ex. The former couple shares two kids, True, 4, and a son, whom they welcomed via surrogate in August. Tristan is also dad to Prince, 6, and Theo, 1.

In addition to Khloe being by Tristan's side, the source says that the reality star's mom, Kris Jenner, and sister, Kim Kardashian, will be on hand for Andrea's funeral.

"A private funeral for his mother is scheduled that Khloe, Kim, and Kris are expected to attend," the source says.

Another source tells ET, "Khloe was very close to Tristan's mother, and as with any partner, friend, etcetera, the family will be first to support them, especially in this very difficult time."

TMZ, who first published news of Andrea's death, reports that she died suddenly after a heart attack. 

Andrea and Trevor Thompson welcomed Tristan in 1991. Tristan has three younger brothers, Dishawn, Daniel and Amari. In a 2016 interview with the NBA, Tristan revealed that he talked to his mom every day.

"Everyone knows that my mom, she’s very dedicated in taking care of her sons -- especially Amari," Tristan said of his youngest brother, who suffers from epilepsy. "But at the same time, you need to make sure that she's OK. It's asking a lot for one parent to do, so I always try to make sure that she's doing well, too. Whether she’s getting a massage or just get out of the house and have a babysitter watch for a couple hours, that's crucial."

Years before, when Tristan was chosen fourth overall in the 2011 NBA draft, Andrea penned an article for the Toronto Star reflecting on how her son made it to the NBA.

"Trevor is a truck driver. I drive a school bus. We work hard. In life you have to work hard to receive your blessing, everybody knows that. I never had it easy as a child," she wrote. "I didn’t have to tell Tristan anything to empower him. Tristan saw us always working hard, and he wanted to get ahead."

As for the advice she had for her son over the years, Andrea said, "Keep going. Don’t look back. Keep fighting."