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8 year old"In the rush I think some of those words didn't come out exactly the way they were meant," Dr Harold Bornstein told NBC News.
Dr Bornstein's letter said Mr Trump, 70, would be the "healthiest individual ever elected to the presidency".
He said he "picked up" the candidate's own "kind of language" when writing it.
Two weeks before the note was written, Mr Trump tweeted to say he had "instructed my long-time doctor to issue, within two weeks, a full medical report", adding that it would "show perfection".
Mr Trump later highlighted the endorsement from Dr Bornstein, who faced questions about its exaggerated tone.
The doctor, of York's Lenox Hill Hospital, says he may have overstated the case "so that they [the Trump campaign] would be happy".
"I think I picked up his kind of language and then I just interpreted it to my own," he said.
Mr Trump would be the oldest person to be elected US president, were he to win in November. Mrs Clinton is 68.
The health of two candidates has become a focal point in the campaign, with Mr Trump repeatedly alleging that his Democratic rival Hillary Clinton is physically unfit.
Mrs Clinton has hit back at what she says are unfounded allegations, and Mr Trump has faced criticism over the personal nature of the attacks.
Neither candidate has divulged their full medical record. Mrs Clinton released a statement from her doctor, Lisa Bardack, last year.
Dr Bardack said: "She is excellent physical condition and fit to serve as President of the United States."
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