Women are more likely to be depressed than men, and a new study suggests their hormones can be to blame. 

A team of researchers from the Medical University of Vienna used cross-sex steroid hormone treatment on 33 transexuals who had been seeking gender reassignment therapy. The treatment's serotonin transporter levels  were measured both before and after the treatment with positron emission tomography (PET). 

When transsexuals undergo gender reassignment therapy they are given high doses of opposite gender hormone therapy to adapt their appearance to that of the other gender. Genetic women are given testosterone, while genetic men are given oestradiol as well as other medications to suppress testosterone production, study author Georg Kranz explained to Red Orbit. 

After the researchers reviewed the patients, they saw a clear connection between testosterone and low levels of depression. 

"The study has shown that testosterone increases the potential binding sites for commonly prescribed antidepressants such as SSRIs in the brain and therefore provides major insights into how sex hormones affect the human brain and gender differences in psychiatric illnesses," Siegfried Kasper, head of the university's Department of Psychiatry and Psychotherapy, told Red Orbit. 

The researchers hope that their findings will help depression patients get better treatment. 

Internationally more than 350 million people suffer from depression, according to the World Health Organization. 

The study was published in the journal Biological Psychiatry.