A post-mortem examination of the 13-metre carcass of the juvenile female whale was carried out by vets from the Agri-Food and Biosciences Institute.
In a report, they said it most likely died of natural causes, but the exact cause is unknown.
They said the carcass was in a very poor condition.
"The skin was peeling and it had a thin blubber layer," the report said.
"The muscle mass along the back was low and the stomach was empty. The body condition of the mammal was considered sub-optimal."
The body has been buried at a landfill site near Portrush.
A fin whale that washed up on Portstewart Beach at the start of the month was dead for some time before it washed ashore, experts have said.