Loren Ruch, head of content for HGTV and cohost of “HGTV House Party,” died Thursday at Mount Sinai in NYC … and his family is clarifying exactly what happened to the beloved media personality.

Dolly Norris, Loren’s mother, tells TMZ … the HGTV fixture was diagnosed with acute myeloid leukemia about a year ago, and treatment for the cancer left him with a weakened immune system. He was hospitalized a few months back for a bowel blockage — and he then caught pneumonia, which ultimately was his cause of death. Dolly tells us he was cancer-free.

Loren’s passing is even more devastating for the family, as just a few weeks ago he seemed to be on the road to recovery … but, Dolly says, he took a turn for the worse and his health declined quickly — including a lung collapsing.

In a memo leaked to the press … Channing Dungey, chair of Warner Bros. Television Group, called Loren “the rare leader who lifted everyone around him.”

Loren’s mother tells us he was also the most humble of people … and he lit up every room he was in — and that’s why Dolly has been inundated with texts and calls about his passing.

We hear the family is planning a small family service in NYC … and then a large celebration of life in L.A., per Loren’s wishes.

He is survived by Dolly; his husband, David Salas; his brother, Geoffrey, his father, Larry; and his stepfather, Will Waterman.

Loren was 55.