Why Garry Shandling’s cause of death is still under investigation 

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Garry Shandling died on Thursday, March 24, and his rep immediately told media outlets doctors believed the TV legend died of a heart attack; however his exact cause of death is still unknown, FOX411 has learned.

The case of Shandling's cause of death is being handled by the Los Angeles County Coroner's Office and they tell us, “An exam was performed and it is pending toxicology [results]…There might be [other medical issues at hand] but we are not releasing any more information until we have all the full reports back.”

We’ve learned Shandling’s death certificate remains unsigned. The Coroner’s Office told FOX411 it is because Shandling had not seen his primary care doctor in over a year at the time of his death.

TMZ reported Shandling complained of shortness of breath and aches to a doctor who he was friends with and who visited him at home on Wednesday. The doctor told him if he wasn’t feeling better he should go to the emergency room Thursday. Shandling called 911 Thursday morning and reportedly collapsed during the call.

“The doctor that he had talked to was not his primary physician,” a rep for the Coroner’s Office told us. “He can’t sign the death certificate because he didn’t see him professionally; he met him a couple of years ago and was just a friend. His primary doctor hadn’t seen him in over a year, so we have to take jurisdiction.”

The star’s longtime publicist Alan Nierob said Shandling had no previous heart issues.

“A death certificate will be issued…by us and when we get all [results and] everything back we will put a final cause,” the rep for the coroner’s office said. “We have talked to a couple doctors and it is an ongoing investigation…None of the doctors have seen him in the last 12 months.”

In January, the 66-year-old comedian spoke with Jerry Seinfeld on “Comedians in Cars Getting Coffee” about a health issue he discovered.

"So it turns out I had a hyper parathyroid gland that was undiagnosed because the symptoms mirror the exact same symptoms an older Jewish man would have, which is, you know -- lethargic, you get puffy, you get heavy, you kind of feel like you want a divorce, but, you know, you're not married.”

On Sunday, a group of Shandling’s celebrity friends played one last basketball pickup game in his honor at his home. Judd Apatow, Kathy Griffin, Sarah Silverman took to social media to remember their friend.