Michelle Trachtenberg Cause of Death Undetermined, Family Refuses Autopsy

Michelle Trachtenberg's cause of death will not be released ... because the beloved actress' family has turned down an autopsy, TMZ has confirmed.

According to the New York City medical examiner ... Michelle's family has asked the ME not to perform an autopsy on Michelle to determine the cause of death.

The medical examiner tells us that if there were any suspicion of criminality, then an autopsy would automatically be performed ... but, since there isn't, they won't be doing one.

As we told you ... Michelle was found unresponsive in her home by her mother, Lana, on Wednesday, February 26. Lana immediately called police and paramedics, but they pronounced Michelle dead at the scene.

Lana saw Michelle alive just hours earlier ... after they parted ways around 10 PM Tuesday. Michelle hadn't been acting like she was having any trouble with her health -- she even went out with friends just a few nights before her untimely passing.

Sources told TMZ ... Michelle had recently had a liver transplant mere months before her passing -- it's unclear if that played a factor in her death.

Dozens of stars shared their fondest memories of Michelle after her passing ... including her "Gossip Girl" costars Blake Lively and Ed Westwick and her "Buffy the Vampire Slayer" pals Sarah Michelle Gellar and Alyson Hannigan.

Michelle was 39.