People with autoimmune diseases may be more likely to develop psychosis, according to new research

What exactly causes autoimmune diseases — where the body mistakenly targets and attacks its own tissues — is a bit of a mystery. But what is known is that if you have one autoimmune disorder, such as type 1 diabetes, an overactive or underactive thyroid, or multiple sclerosis, your chances of developing another one are higher.

The reasons for why this could be are murky, but studies have shown it could be because "people with several autoimmune diseases have a particularly susceptible gene pool," according to Chaim Putterman, an immunology researcher in an article for U.S. News & World Report.

According to the research of Alexis E Cullen, a postdoctoral fellow in the department of psychosis studies at King's College London, people with autoimmune diseases are also more likely to develop psychosis.

In an article for The Conversation, Cullen wrote about previous research that has been inconclusive about whether there is a relationship between autoimmune disorders and psychosis. So in a new study, published in the journal Biological Psychiatry, she and her team decided to conduct a meta-analysis of several studies to see if they could get a clearer picture. They looked at 30 different studies, containing data on 25 million people overall.

The team were particularly interested in autoimmune diseases that target the body as opposed to the brain, so they gathered data from all non-neurological autoimmune disorders. Overall, people with any autoimmune disorder were 40% more likely to have a psychotic disorder, like schizophrenia.

Rheumatoid arthritis date was omitted from the first part of the study because previous research has found there is a negative association, meaning the rates of rheumatoid arthritis are lower in people with psychosis than in the rest of the general population.

When looking at specific disorders in the second part of the study, psychosis was more prevalent in people who had pernicious anaemia (a vitamin B12 deficiency), pemphigoid (a blistering disease), psoriasis (a skin disorder), coeliac disease (gluten allergy), and Graves' disease (a thyroid problem).

In comparison, psychosis was less likely in those who had rheumatoid arthritis and ankylosing spondylitis (arthritis of the spine), giving further evidence that these disorders could protect against psychosis.

Cullen said there are a range of possibilities for the connection, including inflammation. But this wouldn't explain why certain types of arthritis are negatively associated with psychosis.

"Although all autoimmune disorders activate the body's immune system, the exact response differs depending on the disorder," she wrote. "This might go some way to explaining why we found different relationships for individual autoimmune disorders, and suggests that inflammation cannot be the only mechanism."

There may also be a genetic factor. Or there could be involvement from newly discovered antibodies that attack brain cells.

The reasons people develop psychotic disorders is already complicated — researchers don't yet fully understand what causes schizophrenia, for example. There's also evidence of people experiencing psychosis at very high altitudes, such as when climbing Mount Everest.

Whatever the causes, Cullen said the new study provides stronger evidence that a relationship between autoimmune diseases and psychosis does exist. She added that the results will help gain a better understanding of the risk of psychosis because then doctors can monitor people closer for certain symptoms.

"This is important because early intervention has been shown to improve long-term outcomes for people in the initial stages of a psychotic disorder," she said.