US actress Michelle Trachtenberg died a natural death from complications caused by diabetes, according to the New York City medical examiner's office.

The 39-year-old actress was found "unconscious and unresponsive" in her New York City apartment in February.

Officials did not perform a post-mortem examination, but said toxicology tests determined Trachtenberg's cause of death.

The US actress was best known for playing Buffy the Vampire Slayer's younger sister Dawn Summers, and later took on the role of manipulative socialite Georgina Sparks in Gossip Girl as an adult.

Trachtenberg's family, who are Orthodox Jews, had reportedly objected to an autopsy, citing religious reasons.

Because there was no sign of foul play, the medical examiner did not overrule the relatives' objection.

An unnamed source told NBC News the actress had received a liver transplant before her death. The exact timing or reasoning of the operation is unclear.

Trachtenberg's sudden death shocked her fans.

She rose to fame as a childhood star making her debut in a number of Nickelodeon projects including the film Harriet the Spy.

In the early 2000s, she was nominated for several acting awards - including a Daytime Emmy Award - for her role in Buffy the Vampire Slayer.

She also starred in films including EuroTrip, Ice Princess, Killing Kennedy, and Sister Cities.

Following her death, fellow actors paid tribute.

Blake Lively, a Gossip Girl co-star, said everything Trachtenberg did "she did 200%".

"She laughed the fullest at someone's joke... she cared deeply about her work, she was fiercely loyal to her friends and brave for those she loved, she was big and bold and distinctly herself," Lively wrote on social media.