Johnson, 30, an heir to the Johnson & Johnson health care products fortune, died of diabetic ketoacidosis, the coroner said in a brief statement. Diabetic ketoacidosis is a life-threatening condition caused by lack of insulin and sky-high blood sugar.
An autopsy was conducted January 5, a day after Johnson's body was found at her home. The coroner's office said it would not be able to discuss specifics until the autopsy report and toxicology results become available within two weeks.
Dr. Michael Baden, former chief medical examiner in New York City, said Johnson probably slipped into a diabetic coma, but people can usually be resuscitated from that in the first 24 hours.
"If you have somebody with diabetes, there should be somebody checking up on that person," said Baden, who has been an expert witness in cases involving the late comedian John Belushi and former pro football star O.J. Simpson.
A message left for a family spokesman was not immediately returned.
Johnson, the daughter of New York Jets owner Robert "Woody" Johnson, lived a party life. In December, she announced her engagement to bisexual reality TV star Tila Tequila.
Her body was flown east for a private funeral.
Johnson had been an insulin-dependent diabetic since childhood. A memorial fund in her name was set up with the Juvenile Diabetes Research Foundation. Her father is chairman of that organisation and co-author of the book Managing Your Child's Diabetes.
Casey Johnson lived in a quiet neighborhood behind big wooden gates marked "Grumblenot," but she often partied with high school friend and fellow heiress Paris Hilton.
Johnson was charged in December with burglary and receiving stolen property for allegedly taking $22,000 in clothing, jewelry and other items from a friend's home. She pleaded not guilty and faced a February hearing.
She is survived by her parents, two younger sisters, and an adopted child she named Ava-Monroe, after her idol, Marilyn Monroe.
