Actress Michelle Trachtenberg’s death was ruled “undetermined” Thursday morning after her family objected to an autopsy, according to a spokeswoman for the city medical examiner’s office and sources.

The office said it’s obligated to honor the family’s request due to the state’s public health law, the sources said. This confined the medical examiner to conducting an external examination, which limits what can be determined.

Trachtenberg, 39, was found by her mother around 8 a.m. Wednesday at One Columbus Place, a 51-story luxury apartment complex in Manhattan’s Central Park South neighborhood, the sources said.

The actress, whose credits include the title role in the 1996 film “Harriet the Spy,” “Gossip Girl” and “Buffy the Vampire Slayer,” had undergone a liver transplant within the last year, a source told The Post, adding that her body may have rejected the organ.

Her family’s wishes not to conduct a formal autopsy means it’s unlikely any further investigation will be done into her shocking death.

Her death certificate now officially states “undetermined” as both cause and manner, sources said.

Trachtenberg’s mother, Lana Trachtenberg, saw her daughter alive around 10 p.m. Tuesday night — just 10 hours before she found her lying in bed face-up and unresponsive in her Manhattan apartment, sources told The Post.

She was declared dead by EMS and her death is not considered suspicious, officials said.

Just last Thursday, the actress was spotted at a popular New York City restaurant, TMZ reported.

In the months leading up to her death, the actress sparked health concerns among her fans as some of her 800,000 Instagram followers started pointing out her alarmingly frail appearance, which she dismissed.

A close friend of Trachtenberg, photographer and media personality Amanda de Cadenet, claimed in an Instagram post Wednesday evening that Trachtenberg was “recently” hospitalized and was aware she had a “high” chance of dying.

“I will not share the details of our conversations over the last 6 months,” de Cadenet wrote before adding, “but you knew that death was a high possibility and I am only sorry that the outcome was not a better one.”

The post was edited later to omit those details. 

Tributes from Trachtenberg’s co-stars poured in as news spread of her untimely death.

Trachtenberg’s “Buffy” co-star James Marsters, who played Spike, told The Post Wednesday, “We have lost a beautiful soul. Michelle was fiercely intelligent, howlingly funny, and a very talented person.”

“She died much too young, and leaves behind scores of people who knew and loved her,” he added.

Fellow “Buffy” alum Sarah Michelle Gellar, who played main character Buffy Summers, also shared a heartfelt Instagram post.

“Michelle, listen to me, I love you. I will always love you. The hardest thing in this world, is to live in it. I will be brave. I will live … for you,” she captioned — quoting what Buffy says to her sister, Dawn Summers, played by Trachtenberg, in an episode of the show.