Johnson, who was diagnosed with diabetes as a child and had worked with the Juvenile Diabetes Foundation, told the New York Observer in 2000 that she gave herself insulin shots every day.
"She was still suffering from diabetes and she was still on diabetic medications," a police source with knowledge of the situation tells PEOPLE. "The speculation is she came home after a night of partying and passed out without taking her meds or her insulin and that was the end of her."
Police say there was no sign of foul play and, although Johnson, 30, had battled drug problems in the past, there were no obvious signs that she took any illicit drugs before her death.
An autopsy was imminent, although a conclusive report might take several weeks pending the results of toxicology tests.
The source also notes that Johnson has had some wild ups and downs in her life over the past few months. She had become engaged last month to reality star Tila Tequila. And about five weeks ago, she was arrested on suspicion of grand theft for allegedly stealing from a fellow Hollywood party girl.
Johnson, who leaves behind a toddler daughter Ava whom she had adopted, was the great-great granddaughter of the founder of the pharmaceutical giant, and the daughter of New York Jets owner Robert Wood Johnson.