A new study into how Covid-19 affects men has found that testosterone may play a key role, showing that men with low levels of the sex hormone were more likely to end up in intensive care if they became infected.
The German government-funded study, which has not yet been peer reviewed, looked at the first 45 confirmed Covid-19 patients admitted to intensive care at the University Medical Centre Hamburg-Eppendorf.
Thirty-five men and 10 women were studied, with seven needing oxygen and 33 needing ventilation. Nine men and three women died.
This gender balance is in line with previous studies, which have shown that men are more likely to suffer serious complications from Covid-19.
Patients were tested for 12 hormones, including testosterone.
Testosterone plays a key role in the immune system, including its ability to fight viral infection.
This is particularly true for men, because testosterone is the male sex hormone.
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Of the male patients, more than two thirds recorded low levels of testosterone.
The majority of female patients (60 per cent) had elevated testosterone levels.
While low levels of testosterone in men impairs the immune response, the study found that higher testosterone levels in women were linked to a more significant inflammatory response.
Professor Gülsah Gabriel from the Leibniz Institute for Experimental Virology in Hamburg, who was involved in the study, told the Daily Mail: "The majority of male Covid-19 patients had low testosterone levels.
"Of those male Covid-19 patients who died, the majority also had low testosterone levels.
"Thus, low testosterone levels in men seem to be a risk factor for severe and even fatal disease outcome in men upon infection with so-called 'cytokine inducing' respiratory viruses."
Our experience with Covid-19 is showing us that things can go awry when our immune systems push back too hard and inadvertently made us sicker.
One of the most lethal examples of this is what's called a cytokine storm.
When our immune system detects a virus, it can over-react by sending a surge of immune cells and their activating compounds - cytokines - straight into our lungs.
This doesn't just inflame the lungs and build up fluid, causing respiratory distress characterised in Covid-19 by violent bouts of coughing, but also opens the door for contamination by a secondary bacterial pneumonia, boosting the risk of death.
Professor Gabriel told the Daily Mail: "It seems that testosterone has a dampening impact on the virus induced cytokine storm, which finally leads to death in both cohorts.
"Men with normal testosterone levels do not present a cytokine storm and thus are more likely to survive."