The Home Secretary was diagnosed with Type 1 diabetes two months ago and must now inject herself with insulin at least twice a day for the rest of her life.
Commentators had seized on Mrs May's dramatic weight loss over the last 18 months as proof that she was undergoing a style makeover in preparation for a future leadership bid, but the Conservative Cabinet minister said that dropping two stones was partly down to the illness.
"The diabetes doesn't affect how I do the job or what I do. It's just part of life . . . so it's a case of head down and getting on with it," Mrs May said. "It was a real shock and, yes, it took me a while to come to terms with it.
"It started last November. I'd had a bad cold and cough for quite a few weeks," she said. "I went to my GP and she did a blood test which showed I'd got a very high sugar level – that's what revealed the diabetes.
"The symptoms are tiredness, drinking a lot of water, losing weight, but it's difficult to isolate things. I was drinking a lot of water. But I do anyway.
"There was weight loss but then I was already making an effort to be careful about diet and to get my gym sessions in.
"Tiredness – speak to any politician and they will tell you the hours they work. Tiredness can be part of the job. It is full on."
Pressed on whether the illness would prevent her one day succeeding David Cameron, she said: "There is no leadership bid. We have a first-class Prime Minister and long may he continue."
Doctors told Mrs May she had the condition, which means her body does not produce insulin, in November but initially they thought she had Type 2 diabetes.
"It doesn't and will not affect my ability to do my work. I'm a little more careful about what I eat and there's obviously the injections, but this is something millions of people have . . . I'm OK with needles, fortunately," she said. "There's a great quote from Steve Redgrave who was diagnosed with diabetes before he won his last Olympic gold medal. He said diabetes must learn to live with me rather than me live with diabetes. That's the attitude."
l Scottish bottled water company Purely Scottish is to donate 10p from every six-pack of water sold in Tesco stores to Diabetes UK, with the aim of donating more than £100,000 to fund support and research into the condition.
John Wedderburn, owner of the water firm, said: "Purely Scottish is delighted that Tesco is giving us a chance to contribute to this wonderful charity."