Public health officials are scrambling to understand long COVID, the condition in which patients report symptoms like fatigue, muscle weakness, and “brain fog” months after infection. A new study released Wednesday shows one way doctors might diagnose who has the chronic condition and indicates a possible way to treat it.

Researchers from the Yale School of Medicine and the Icahn School of Medicine at Mount Sinai found that long COVID patients had half the levels of cortisol—a hormone that guides the body’s response to stress—as uninfected individuals and those who had recovered fully from COVID. The study, which has yet to be peer reviewed, analyzed 215 individuals, 99 of whom were long COVID patients.

If the study’s findings are corroborated, low cortisol levels could help doctors determine who has long COVID, allowing for better treatment. It might also help public health officials better understand how widespread the chronic condition is as long COVID threatens to pull millions of workers from the U.S. labor force.

Scientists are still trying to understand the myriad ways COVID affects the human body. In addition to fatigue and muscle weakness, COVID has also been connected to higher rates of depression, hair losscardiovascular disease, and heart attacks. An earlier study found that even a mild case of COVID led to a decline in brain tissue equivalent to a decade’s worth of aging. And last year, a study found that COVID attacked fat cells, perhaps explaining why overweight and obese individuals are more at risk for severe COVID.

What is cortisol?

Cortisol is a hormone that helps regulate bodily functions like blood pressure, digestion, and sleep cycles as a person experiences stress. Amid stress, cortisol levels rise, encouraging the body to increase the brain’s use of glucose and release substances that repair tissue, while suppressing bodily functions that are nonessential in a dangerous situation.

Having less of a stress hormone might sound like a good thing, but abnormally low levels of cortisol have been connected to symptoms like muscle weakness, persistent fatigue, a loss of appetite, and low blood pressure.

Researchers have reported low cortisol levels in those with chronic fatigue syndrome, another long-lasting medical condition whose causes and symptoms have puzzled scientists. Studies have found that low-dose cortisol treatments have helped patients with chronic fatigue.


Governments are quickly realizing that long COVID presents a threat to the economy. The U.S. government is spending $1.2 billion to determine the causes behind the condition and research possible treatments.

The U.S. Centers for Disease Control and Prevention estimates that one in five American adult COVID-19 patients is suffering from symptoms associated with long COVID. Katie Bach, a senior fellow at the Brookings Institution and an expert on long COVID’s effect on the workforce, estimates that up to 4 million workers, or 2.4% of the U.S. labor force, is out of the workforce as a result of long COVID. Other economies are reporting even higher rates of the condition, with the U.K. reporting in April that long COVID was hurting 4% of the country’s workforce.

Wednesday’s study may help doctors treat long COVID. Akiko Iwasaki, one of the study’s authors, told Bloomberg that the research provides “many clues for therapeutic avenues, including antivirals and hormone therapy.”

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