For a relatively common ailment, endometriosis is rather mysterious. Its symptoms are generic, and common to many disorders; the only definitive diagnosis can usually only come from a biopsy. But new research published in the journal Human Reproduction suggests fatigue should be given more weight as a symptom of endometriosis.

Endometriosis is a condition in which endometrial cells that usually form inside the uterus also grow abnormally outside of it, in other areas of the pelvic region, such as ovaries, the abdominal cavity and Fallopian tubes. The more prominent symptoms include pain and fertility problems, although many women with endometriosis do not experience these symptoms. The cause of endometriosis remains unknown, but globally, 6 to 10% of women have the condition.

The international study involved more 1120 women recruited from hospitals and private practices in Switzerland, Germany and Austria between 2010 and 2016. The researchers found the experience of fatigue was doubly common among women who had endometriosis, when compared to those who didn’t have the condition. Even after taking into account factors such as insomnia, pain, occupational stress, BMI and motherhood – all factors that affected fatigue levels — the results were still the same.

This means that endometriosis causes fatigue in the body irrespective of other factors that may also result in fatigue, explains research lead Brigitte Leeners, a professor and deputy head of reproductive endocrinology at University Hospital Zurich, Switzerland.

“Although chronic fatigue is known to be one of the most debilitating symptoms of endometriosis, it is not widely discussed and few large studies have investigated it,” Leeners says. “We believe that in order to improve the quality of life for women with this condition, investigating and addressing fatigue should become a routine part of medical care, and doctors should investigate and address this problem when they are discussing with their patients the best ways to manage and treat the disease. It would also help these women if steps were taken to reduce insomnia, pain, depression and occupational stress.”

Leeners’ team found 50.7% of women diagnosed with endometriosis had frequent bouts of fatigue in comparison to 22.4% of women without the condition. Among women with endometriosis-related fatigue there was also a seven-fold increase in insomnia, a four-fold increase in depression, a two-fold increase in pain and an almost 1.5-fold increase of occupation stress. The age, stage or time when the condition was diagnosed was not linked to fatigue.

Researchers believe the reason endometriosis could lead to fatigue is because endometrial lesions lead to inflammation which in turn triggers the immune system. Proteins that are utilized when the immune system is active also play a role in fatigue. Furthermore, regular, high stress on the body — from, say, chronic inflammation — can contribute to adrenal fatigue.

Researchers are calling for clinicians to give more consideration to fatigue as a symptom of endometriosis, though no changes in diagnostic standards have changed. Still, this might be a much needed step forward toward better diagnosis of an already difficult-to-spot condition.