The 53-year-old announced her departure in her Sunday People column, which she wrote before entering the Celebrity Big Brother house on Thursday (August 22).
"I'm not returning to ITV's lunchtime talk show Loose Women, the job I've done for more than a decade. When it returns on September 9 for its 18th series I will no longer be sitting on the panel," McGiffin said.
"I am sure that some of you will rejoice at this news but, I think, those who might not be so pleased deserve some sort of explanation. I'd been thinking about it for at least a year.
"It seems totally mad to quit one of the best jobs in the world - and it is one of the best jobs in the world - where you chat for an hour on television with like-minded women and get paid reasonably well for it.
"But I felt as if I'd completely plundered my private life and exhausted my repertoire of anecdotes to the point of actually boring myself. I hadn't realised that after nearly 13 years, I'd appeared on 1,172 episodes of Loose Women.
"So it wasn't an easy decision and it certainly wasn't made lightly. I can't say I've enjoyed every single minute but I've had so many laughs and made so many friends. The show has been such a big and important part of my life for so long.
"I will miss everyone a lot, especially the behind-the-scenes teams who over the years have all worked so hard. But mostly life will be weird without the daily banter with my mates, the other women, though I'm sure that we will stay in touch."
McGiffin, who was the longest-serving panel member, is now looking for "new avenues and fresh adventures", but insisted that doing Celebrity Big Brother - which she has ruled out in the past - had nothing to do with quitting Loose Women.
It had been rumoured that McGiffin was facing the axe from the show as producers look to freshen it up to boost ratings.