Casey Johnson died from complications related to her diabetes, the Los Angeles Coroner's Office told TVGuide.com.
The Johnson & Johnson heiress died of diabetic ketoacidosis, a life-threatening condition caused by a lack of insulin and high blood sugar, chief coroner Ed Winter said.
Casey Johnson dead at 30
"She wasn't taking her insulin. She wasn't taking good care of herself," he said.
The cause of death has been ruled natural, and a full autopsy report will be completed in two weeks, Winter said.
Johnson was diagnosed with juvenile diabetes as a child, and co-wrote a book with her father, New York Jets owner Woody Johnson, called Managing Your Child's Diabetes. Woody Johnson also established the Juvenile Diabetes Research Foundation, which his daughter supported.
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Johnson was found dead at the age of 30 at her Los Angeles home on Jan. 4. Winter said her exact date of death has not yet been determined.