Interestingly, low testosterone appears to actually be a risk factor for developing MS. In fact, men with hypogonadism have a risk ratio of 4.62 of developing MS at a future date. So men with low testosterone are almost 5 times more likely to get MS than men with normal testosterone.

In addition, testosterone treatment has been shown to be immunomodulatory and to have a protective effect against autoimmune diseases such as lupus, arthritis, and diabetes. It has also been proven to be protective of neurons in the CNS in autoimmune encephalitis. The science on this is clear at this point.Testosterone treatment has also been proven to slow the progression of Alzheimer's by acting on neural androgen receptors. Testosterone has an effect in protecting neurons in culture against glutamate-induced toxicity and oxidative stress, and stimulates myelin formation and regeneration mediated through the neural androgen receptor.

Extrapolating on this, testosterone therapy would likely be beneficial against MS, as it is both an autoimmune process that involves the central nervous system.This study, published by JAMA in 2007 used testosterone gel to increase testosterone levels in 11 men with RRMS. The results were improvement in muscle mass and bone density (obviously), but also in cognition on the Multiple Sclerosis Functional Composite score. Most importantly, testosterone therapy significantly slowed brain atrophy compared to the non-treatment arm. The longer subjects were on the therapy, the greater neuroprotective effect.

TL;DR: Low testosterone appears to be a significant risk factor for MS, and TRT shows great promise as a treatment. So far, limited studies have shown increased cognition of those with MS on TRT, and slower progression of the disease.