A new US study could finally help answer why men are more likely to die from Covid-19 than women. The study, by researchers from the Washington University School of Medicine in St Louis, Missouri, revealed that men with low testosterone levels are more likely to require intensive care.

However, the study, published in the journal JAMA Network Open Tuesday, does not prove that severe Covid is caused by low testosterone. Instead, the researchers establish a link between severity of the disease and low levels of the sex hormone, and urge caution against ongoing clinical trials that are investigating hormonal therapies — which block or lower testosterone or increase oestrogen — as a treatment for men with Covid.

“During the pandemic, there has been a prevailing notion that testosterone is bad,” Abhinav Diwan, a professor of medicine at the university, said in a statement.

“But we found the opposite in men. If a man had low testosterone when he first came to the hospital, his risk of having severe Covid-19 — meaning his risk of requiring intensive care or dying — was much higher compared with men who had more circulating testosterone. And if testosterone levels dropped further during hospitalisation, the risk increased,” Diwan added.

For the study, researchers measured several hormones in blood samples from 90 men and 62 women, who came to Barnes-Jewish Hospital in St Louis with symptoms of Covid-19 or confirmed cases of the illness. Of these, 143 were hospitalised.

The researchers measured hormone levels after three, seven, 14 and 28 days — depending on how long the patient was hospitalised.

In addition to testosterone, the researchers also measured levels of oestradiol, a form of oestrogen produced by the body, and IGF-1 — an important growth hormone that is similar to insulin and plays a role in maintaining muscle mass.

Among men, only testosterone levels were linked to Covid-19 severity. At hospital admission, men with severe Covid had average testosterone levels of 53 nanograms per decilitre, and men with less severe disease had average levels of 151 nanograms per decilitre.

A blood testosterone level of 250 nanograms per decilitre or less is considered low testosterone in adult men. By day three, the average testosterone level of the most severely ill men was only 19 nanograms per decilitre.

The researchers also found that those with the lowest levels of testosterone in the blood were at highest risk of going on a ventilator, needed intensive care or were dying.

Thirty-seven patients, 25 of whom were men, died over the course of the study. Meanwhile, no such link between levels of any hormone and disease severity was found among women.