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The 37-year-old Take That singer was told by medical experts his feelings of lethargy stemmed from a lack of the hormone - which boosts a man's sex drive - and says the supplementary injections have changed his life.
He told Britain's Esquire magazine: "I went to the doctor to get some Human Growth Hormone (HGH).
"It's what all the old fellas are on out there in Los Angeles that's making them look 40 instead of 60.
"It's improving their health, their memory, their hair, skin.
"Could give you cancer. I weighed that up. Thought I'd have it anyway. Went to see a Hollywood doctor. Had my blood tests. Went back. He said, 'You don't need HGH. You've got the testosterone of a 100-year-old man.'
"And then everything made sense. It was kind of an epiphany that day."
Tempted to cheat
Robbie married US actress Ayda Field last August and insists he will always remain faithful to her - though he admits he could be "tempted" to cheat.
When asked if he could ever see himself cheating on Ayda, he replied: "Actually, no, because I don't want to break Ayda's heart. That's the last thing I want to do. I met Ayda, I fell in love, that's what happened."
The Angels hitmaker also spoke of his belief that people naturally want to cheat and branded marriage "made-up bull".
He said: "The rules aren't set up right. Because people are f***ing outside of their marriages, outside of their relationships. I think that means we're built to f***.
"And marriage, that whole institution, is made-up bull."
