As Hillary Clinton prepared to head out on the campaign trail on Thursday, her personal doctor released a letter saying the US presidential candidate is responding well to treatment for pneumonia.

The letter also provided a trove of other health details about the former secretary of state, who apparently hoped releasing the letter would put concerns about her health to rest just three days after the pneumonia diagnosis sidelined her on Sunday.

At nearly the same time Clinton released the letter, Donald Trump, her Republican opponent in the race for the White House, revealed results of a physical examination he underwent last week. Though less detailed, the information also appeared aimed at answering questions about how transparent he has been about his health.

There is heightened interest in details about the health of both candidates because they are the oldest to seek the highest office in the United States and because they have been accused of being secretive or ambiguous about their physical health.

Clinton's doctor, Lisa Bardack, described in the two-page letter released Wednesday how Clinton became "overheated and dehydrated and as a result felt dizzy" on Sunday during memorial service in New York for the September 11, 2001, terrorist attacks.

"I examined her immediately upon her return home; she was re-hydrating and recovering nicely," said Bardack. "Iadvised her to stay home and rest for the next several days. Mrs Clinton has since been evaluated by me several times and continues to improve."

Clinton has been treated with antibiotics, said Bardack, who characterized the illness as a "mild non-contagious bacterial pneumonia."

The letter also revealed that Clinton takes the blood thinner Coumadin and a common medication for hypothyroidism. Bardack also explained in detail how Clinton, 68, developed symptoms of sinusitis and an ear infection in January, and soon afterward had a tube placed in her ear, which improved her symptoms.

Her cholesterol, blood pressure, heart rate and body temperature were all normal and "she is in excellent mental condition," Bardack added before concluding the letter by saying Clinton "continues to remain healthy and fit to serve as president of the United States."

Trump, 70, released his health data in a more dramatic way: during the taping of the Dr Oz television show. Asked by host Dr Mehmet Oz whether he would release his medical records, he surprised him by saying "Well, I really have no problem in doing it. I have it right here." He then asked the audience, "Should I do it?"

Trump's campaign had previously said it planned to share the information on Thursday, but it began to leak out shortly after the show was taped.

The report was from a physical he had last week with his long-time personal doctor, Harold N Bornstein. It revealed that he takes a drug to lower cholesterol and with a weight of about 107 kilograms and height of 187 centimetres, he is about 13 kilograms overweight according to standards commonly used by doctors.

Trump reportedly told Oz he would like to lose about 7 kilograms, but otherwise said, "I feel as good today as I did when I was 30."

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