How Knowing About APS-3B Led Doctors to Two Hidden Diagnoses in a Patient with Hashimoto's
Autoimmune Polyglandular Syndrome Type 3, known as APS-3, is defined by the presence of autoimmune thyroid disease alongside at least one other autoimmune condition, with the key distinction that Addison's disease is not part of the picture (its inclusion would shift the diagnosis to APS-2). APS-3 is further divided into four subtypes based on which additional conditions are present. APS-3A pairs thyroid autoimmunity with type 1 diabetes. APS-3B combines it with gastrointestinal autoimmune diseases such as autoimmune gastritis or primary biliary cholangitis. APS-3C includes vitiligo or alopecia alongside thyroid disease, and APS-3D involves neurological autoimmune conditions. This classification system is not just academic labeling; it is meant to serve as a clinical roadmap, guiding doctors to look for related conditions that tend to cluster together.
A case report published in May 2025 in the European Journal of Case Reports in Internal Medicine demonstrates exactly how powerful that roadmap can be when applied thoughtfully. Researchers from Shimane University Hospital and Kyoto University Hospital in Japan describe an 84-year-old woman with a longstanding history of Hashimoto's thyroiditis who came to medical attention with forgetfulness. When endoscopy was performed, it revealed pan-atrophic gastritis, a widespread thinning and loss of the stomach lining. Pan-atrophic gastritis is commonly caused by chronic Helicobacter pylori infection, and it might easily have been attributed to that cause.
Instead, the clinical team paused to consider the APS-3B framework. Because the patient had Hashimoto's disease, they recognized that autoimmune gastritis, rather than a bacterial infection, was a plausible explanation. Two specific endoscopic clues supported this reasoning: the presence of adherent mucus on the stomach lining and remnants of oxyntic mucosa, the specialized acid-producing tissue that autoimmune gastritis tends to spare in distinctive patterns. Guided by these observations, the physicians ordered tests for anti-endogenous antibodies. The results confirmed autoimmune gastritis, a condition in which the immune system attacks the stomach's parietal cells, destroying the ability to produce stomach acid and the intrinsic factor needed to absorb vitamin B12.
The APS-3B framework then prompted the team to take one further step. Primary biliary cholangitis (PBC) is a chronic autoimmune liver disease in which the immune system progressively destroys the small bile ducts within the liver. PBC is known to associate with autoimmune thyroid disease, placing it within the APS-3B cluster. The physicians tested for anti-mitochondrial M2 antibody, the key diagnostic marker for PBC, and the result came back positive. The patient had asymptomatic PBC, a condition that would almost certainly have gone undetected if the clinical team had not been working within the APS-3B diagnostic framework.
The implications of this case are practical and broadly applicable. Many people with Hashimoto's thyroiditis, the most common autoimmune thyroid condition in the world, may carry additional autoimmune conditions that are silent or whose symptoms are attributed to other causes. Digestive complaints like bloating, low stomach acid symptoms, or fatigue may be dismissed as general aging or attributed to the thyroid condition itself, when in reality they may reflect underlying autoimmune gastritis or liver disease. Applying the APS-3B lens gives clinicians a systematic reason to test further rather than stop at the most obvious explanation. For patients, this case is an encouragement to mention all symptoms to their care team, even seemingly unrelated ones, and to ask about screening for companion autoimmune conditions if you have been diagnosed with Hashimoto's or Graves' disease.