Testosterone. Source of prostates and testes, muscles and machismo, chest hair, and according to some, even math skills. Its levels are only one of the biological differences between males and females, but they may help to explain another: the discrepancies in the incidence of autoimmune diseases.
Women are three to nine times more likely than men to suffer from autoimmune diseases, including multiple sclerosis (MS), Grave’s disease, celiac disease, systemic lupus erythematous, and rheumatoid arthritis. Not only do women get these diseases at higher rates, they usually get them at younger ages.
Men’s higher testosterone levels—about seven to eight times higher than women’s—have been shown to be protective for MS in both mice and men. But it was not clear exactly how this worked. Recent work in a mouse model of MS has filled in the downstream effectors that mediate testosterone’s protective effects. These effectors might be useful as therapeutics, whereas testosterone use really isn’t, especially for women, who are the ones who need it most.