The Internet is awash with rumors about Hillary Clinton's health, but there is one medical condition she is actually known to have.

That is hypothyroidism, a condition affecting about 10 million Americans in which the thyroid is underactive, resulting in a variety of symptoms including fatigue, weakness and weight gain.

For most patients, the symptoms can be significantly reduced or eliminated with the use of hormone therapy. Clinton is being treated with a medication called Armour Thyroid, according to her doctor, who has issued a letter saying she's in "excellent" physical condition and is fit to serve as president.

Yet Armour Thyroid is a less common medication used in patients with hypothyroidism, possibly indicating Clinton wasn't responding well to the conventional treatment.

When patients are diagnosed, they typically are given a single hormone replacement called levothyroxine. It is effective with about 90 percent of patients, according to Hossein Gharib, a doctor at the Mayo Clinic and former president of the American Association of Clinical Endocrinologists.

But for the 10 percent of patients who don't find relief from their symptoms, doctors often prescribe a combination hormone therapy such as Armour Thyroid, which uses the T3 and T4 hormones for a more effective medication.

"Armour Thyroid … is a good preparation, but we don't use it that much because most patients are happy with levothyroxine," Gharib said. "The theory is that one hormone may not be enough; two is better."

That idea is supported by research. A 1999 study published in the New England Journal of Medicine found that treating patients with both hormones instead of just one may improve their mood and neuropsychological function.

Despite the evidence, Clinton's use of Armour Thyroid has earned her criticism, most recently from celebrity doctor Drew Pinsky. Pinsky, who is an internist as well as an addiction medicine specialist, said last week it's "weird" Clinton is being treated with what he views as an "unconventional" and "outdated" medication.

"It's something we used to use back in the '60s," Pinsky said in KABC-AM's "McIntyre in the Morning." "What is going on with her healthcare? It's bizarre."

Gharib said the way Clinton's hypothyroidism is being treated isn't a cause for concern.

"She is perfectly capable and in good health, taking her medication," he said. "This is in no way an impediment or deterrent. She is a normal human being at this point."

In the last few weeks of ramped-up debate over the state of Clinton's health, her critics haven't talked about her actual diagnosis, but instead have made vague claims that her energy is lacking, that she's weak and that she can't communicate anymore because of a brain injury.

Republican presidential nominee Donald Trump and his staffers have leveled such accusations, while Breitbart News has sought to keep similarly unfounded claims about Clinton's healthcare alive. Former New York Mayor Rudy Giuliani said over the weekend she looks "tired" and "sick."

The Clinton camp has pushed back hard. Last week, her campaign called the allegations "deranged conspiracy theories." Clinton poked fun at the allegations during a Monday night appearance on the Jimmy Kimmel.

"Take my pulse while I'm talking to you," Clinton told Kimmel, offering him her wrist.

She said attacking her health has become one of Trump's "themes," saying it's part of a "wacky strategy."

Kimmel responded by pulling out a jar of pickles, which Clinton pretended to have trouble opening, to laughter from the audience.

Clinton hasn't released her full medical records — something political candidates don't typically do — but the letter from her doctor goes into more detail than an assertion by Trump's doctor that he's in excellent health.

Medical professionals frown upon those who make unsubstantiated claims about a political candidate's health. Last month, the American Psychiatric Association issued a reminder of its stance against making undiagnosed claims, warning people against trying to "psychoanalyze" the candidates.