The Take That star has refused to disclose what his condition actually is, but admits it leaves him feeling tired and emotional.
‘I thought I was lethargic and depressed as a person,' Robbie, 36, told Magic FM. ‘But I found out a few months ago that I'd been ill, completely by mistake.
'I don't want to go into the whys and wherefores but what I had made you lethargic and made you depressed and I'd had it for years and not known.
'I'm on treatment for it. It's changed my life.'