Endometriosis and thyroid disorders are frequently evaluated together within the context of reproductive health—particularly concerning infertility, recurrent pregnancy loss, and assisted reproductive technologies (ART). However, current evidence does not demonstrate that thyroid dysfunction causes endometriosis, nor that endometriosis directly causes thyroid disease.
According to a recent narrative review, this association is best characterized as a phenotype-dependent reproductive immuno-endocrine interplay. Autoimmune thyroid conditions, hormonal status, immune dysregulation, genetics, and environmental exposures may co-occur in certain patients, yet these factors do not form a simple, unidirectional cause-and-effect relationship.
Interconnected Yet Distinct Thyroid Parameters
The term “thyroid disease” encompasses several distinct biological states that should not be grouped into a single diagnostic entity:
Overt Hypothyroidism: Elevated thyroid-stimulating hormone (TSH) combined with low free thyroxine (FT4).
Subclinical Hypothyroidism: Elevated TSH in the presence of normal FT4.
Hyperthyroidism: Suppressed TSH accompanied by elevated FT4 and/or free triiodothyronine (FT3).
Thyroid Autoimmunity: Thyroid peroxidase antibodies (TPOAb) and/or thyroglobulin antibodies (TgAb), frequently associated with Hashimoto’s thyroiditis.
TSH Receptor Antibodies (TRAb): The hallmark characteristic of Graves’ disease.
While these conditions frequently overlap, they are not interchangeable. An individual may test positive for thyroid autoantibodies while remaining entirely euthyroid (normal thyroid function), or present with thyroid functional abnormalities in the absence of a detectable autoimmune process.
This distinction is essential in endometriosis research. The most consistent epidemiological signal is linked to thyroid autoimmunity (specifically seropositivity for TPOAb and TgAb) rather than isolated variations in TSH, FT4, or FT3. Nevertheless, data across study populations remain heterogeneous and do not demonstrate that thyroid antibodies trigger the formation of endometriotic lesions.
Biological Plausibility
Endometriosis is an estrogen-dependent, chronic inflammatory condition characterized by the growth of endometrium-like tissue outside the uterine cavity. While retrograde menstruation (the backward flow of menstrual debris through the fallopian tubes into the peritoneal cavity) frequently facilitates this process, the establishment and survival of endometriotic implants require permissive host factors, including impaired immune clearance, chronic inflammation, and hormonal dysregulation.
The thyroid gland and the female reproductive tract are closely interconnected. Thyroid hormones regulate systemic metabolism, gonadotropin secretion, sex steroid actions, and endometrial receptor sensitivity.
Experimental studies demonstrate that ectopic endometriotic lesions exhibit increased expression of thyroid-stimulating hormone receptors (TSHR) and thyroid hormone receptor alpha-2 (TRα2), alongside alterations in local thyroid hormone-metabolizing enzymes. In in vitro models, TSH, T4, and T3 have been shown to stimulate the proliferation of endometriotic cells and enhance the production of reactive oxygen species (ROS).
However, these findings remain preliminary. It is not yet established whether altered local thyroid signaling initiates lesion development or represents a secondary tissue response to chronic pelvic inflammation.
Immune Overlap
Immune dysregulation represents one of the most plausible mechanistic links between endometriosis and thyroid autoimmunity.
Autoimmune thyroid disease involves targeted immune destruction of thyroid tissue. In parallel, endometriosis is characterized by altered immune cell trafficking, elevated inflammatory cytokine production, defective clearance of ectopic tissue, impaired macrophage and natural killer (NK) cell cytotoxicity, and complement system activation.
This overlap suggests that certain patients may share an underlying systemic “immune vulnerability” that increases the risk for both disorders. However, this does not indicate that thyroid antibodies cross-react with or directly attack endometrial tissue. Direct antigenic cross-reactivity between thyroid antigens and endometriotic lesions has not been established. Therefore, TPOAb and TgAb should be interpreted as markers of thyroid autoimmunity—or broader immune dysregulation—rather than direct pathogenic mediators of endometriosis.
Particular caution is warranted regarding TSH receptor antibodies (TRAb). While earlier literature suggested a potential elevation of TRAb in endometriosis, modern studies have failed to replicate this association. TRAb is not a valid biomarker for endometriosis and should only be ordered for standard clinical indications, such as suspected Graves’ disease or specific pregnancy-related evaluations.
Understanding the Epidemiological Data
Cohort and registry-based studies show that women with endometriosis have a higher prospective incidence of developing thyroid disorders over time. While longitudinal data help clarify temporality (i.e., endometriosis preceding thyroid diagnoses), they cannot completely eliminate confounding factors or surveillance bias.
Patients diagnosed with endometriosis undergo frequent specialist visits, comprehensive fertility workups, repeated pelvic imaging, surgical procedures, long-term hormonal treatments, and regular medical monitoring. This intensive medical engagement significantly increases the likelihood of incidental thyroid testing, leading to higher detection rates of mild or subclinical thyroid disorders that might otherwise remain undiagnosed in the general population.
Based on current evidence, researchers propose several non-mutually exclusive explanations:
Specific thyroid profiles may influence distinct phenotypic subtypes of endometriosis or infertility.
Endometriosis-driven systemic inflammation and its treatments may influence the presentation of thyroid disorders.
Both conditions may stem from shared genetic, endocrine, environmental, or epigenetic susceptibilities.
The observed association may be heavily amplified by clinical surveillance bias.
Consequently, current data do not support a single dominant mechanism across all patients.
Insights from Genetic Studies
To assess whether a true causal link exists, researchers utilize Mendelian randomization (MR), an analytical method that uses genetic variants as instrumental variables to test for causal effects in observational data. While MR mitigates certain confounding factors, it relies on strict instrumental variable assumptions and remains susceptible to weak instrument bias, pleiotropy, and population stratification.
Recent bidirectional Mendelian randomization studies present nuanced, non-definitive findings:
One study found that genetically predicted higher free thyroxine (FT4) levels were inversely correlated with overall endometriosis risk, with modest, variable associations observed across specific TSH categories, hypothyroidism, and hyperthyroidism. However, the same analysis found no evidence that genetic liability to endometriosis alters thyroid function.
Another investigation supported by clinical infertility datasets found no evidence that a genetic predisposition to autoimmune hypothyroidism increases the risk of developing endometriosis.
Taken together, genetic studies do not support a straightforward, linear causal pathway from thyroid dysfunction to endometriosis. While these data generate hypotheses regarding subtype-specific interactions, they remain insufficient to alter routine clinical decision-making.
Practical Approach and Clinical Recommendations
Current evidence does not support universal thyroid screening based solely on a diagnosis of endometriosis. Clinical guidelines for endometriosis do not recommend routine thyroid function testing as a standard diagnostic requirement or as a disease-modifying intervention.
Thyroid evaluation remains warranted when standard clinical indications are present, including:
Infertility or planned assisted reproductive technology (ART) procedures;
Recurrent pregnancy loss;
Menstrual cycle irregularities or ovulatory dysfunction;
Signs or symptoms suggestive of thyroid dysfunction;
A documented history of abnormal thyroid function tests;
A personal or family history of thyroid disorders or systemic autoimmune disease.
In these clinical scenarios, initial laboratory evaluation consists of serum TSH and FT4. Testing for TPOAb and TgAb is indicated when autoimmune thyroiditis is suspected or when evaluating risk stratification in the context of reproductive planning.
Source: Frontiers
