Endometriosis and Hashimoto's thyroiditis: two distinct facets of a same immune disorder?

Endometriosis is a benign gynecologic disorder that affects 5–10% of women of reproductive age worldwide and is characterized by the presence of endometrial cells outside the endometrium. Women with endometriosis share several symptoms with autoimmune disease patients such as chronic pain, inflammation, fatigue, infertility etc…
A cross sectional survey involving n=3680 patients (with 95% of Caucasian women, 91% being of reproductive age), all diagnosed surgically with endometriosis, showed a strong association of autoimmune diseases with endometriosis. In particular hypothyroidism such as Hashimoto's thyroiditis (HT) showed a 6.5 fold-increase in the endometriosis versus control groups.
Hashimoto's disease is a thyroid dysfunction where your immune system attacks your thyroid, a small gland at the base of your neck. The resulting inflammation leads to a destruction of thyroid cells therefore the thyroid gland becomes hypoactive (hypothyroidism).
Interestingly, a strong correlation was shown between endometriosis and HT regarding their pathogenesis. The two diseases have been associated with a Th1-cell associated cytokines profile. Two factors seem to play a key role in endometriosis and HT development:
IL-18 and IFN-γ.
Endometriosis is characterized by an altered IFN-γ and IL-18 production. IFN-γ and IL-18 gene polymorphism have been connected with the risks of developing endometriosis and IL-18 levels have been correlated with syndromes gravity. They both lead to an inflammatory state facilitating endometriotic lesions adherence.
IL-18 gene polymorphism has also been related to a higher risk of HT. In the thyroid gland of HT patients, IL-18, a potent pro-inflammatory cytokine is up-regulated. IL-18 induces IFN-γ production by Th-1 cells. Strikingly, there is a feedback loop between IL-18/IFN-γ creating an amplification in the secretion of these pro-inflammatory cytokines. These events promote a local immune environment that ultimately leads to the thyrocytes (thyroid cells) destruction. Endometriosis and Hashimoto's thyroiditis have a common etiology suggesting that they are two different expressions of a similar dysregulation in the IL-18/IFN-γ pathway.