Achalasia and Thyroid Disorders: A Hidden Autoimmune Overlap? Epidemiology, Mechanisms, and Clinical Relevance of an Emerging Association

Background: Achalasia is a rare primary esophageal motility disorder characterized by impaired lower esophageal sphincter relaxation and progressive loss of peristalsis. Although its pathogenesis remains incompletely understood, autoimmune mechanisms have been repeatedly proposed. Thyroid disorders, particularly autoimmune thyroiditis and Graves’ disease, have been reported as frequent comorbidities, suggesting a shared autoimmune background. Methods: We conducted a narrative review of PubMed, Scopus, and Web of Science from January 2005 to August 2025. Eligible studies included observational cohorts, case–control analyses, and case reports describing thyroid disease in achalasia. Mechanistic and immunological studies relevant to thyroid autoimmunity were also considered. Data were synthesized narratively and summarized in tables and figures. Results: Despite heterogeneity, evidence consistently indicates an increased prevalence of thyroid disease in achalasia. Early reports described dysfunction in up to one quarter of cases, while Romero-Hernández et al. demonstrated a threefold higher risk of autoimmune thyroid disease. Multicenter data confirmed thyroid autoimmunity in about one fifth of patients. Although thyroid disease did not alter short-term procedural outcomes, unrecognized dysfunction may complicate postoperative evaluation. Immunological findings, including human leukocyte antigen (HLA) susceptibility and lymphocytic infiltration of myenteric plexus, further support a shared autoimmune predisposition. Conclusions: Thyroid disorders, particularly autoimmune hypothyroidism, are more common in achalasia than in the general population. Although the evidence remains limited, the consistent signal suggests a non-random association. Early recognition may improve patient management, while prospective multicenter studies are needed to clarify causality and to determine whether achalasia should be considered part of a broader autoimmune spectrum.