Adult-Onset Still's Disease (AOSD) Complicated by Haemophagocytic Lymphohistiocytosis (HLH) and Transient Thyrotoxicosis: A Diagnostic Challenge.

Adult-onset Still's disease (AOSD) is a rare systemic autoinflammatory disorder that can mimic infectious, autoimmune, and endocrine conditions. Overlapping features may delay diagnosis, as in this case, which was further confounded by abnormal thyroid function tests. A 20-year-old woman presented with persistent fever, migratory arthralgia, myalgia, and a widespread urticarial rash following a sore throat. Initial work-up suggested thyrotoxicosis with weakly positive anti-TSH receptor antibodies, leading to treatment for presumed Graves' disease. Despite therapy, she experienced recurrent admissions with worsening rash, polyarthritis, cytopenia, and seizures. Serial investigations revealed markedly elevated inflammatory markers and ferritin, evolving splenomegaly, and biochemical features of systemic inflammation. Autoimmune and infectious screens were negative. She fulfilled the criteria for AOSD complicated by haemophagocytic lymphohistiocytosis (HLH). Treatment with anakinra and dexamethasone achieved marked clinical improvement, with subsequent normalization of hematological and thyroid profiles. AOSD with HLH is a life-threatening overlap syndrome requiring urgent recognition and treatment. Markedly elevated ferritin levels and multi-system involvement are pivotal diagnostic clues. Thyroid dysfunction, including transient thyroiditis, may occur in systemic inflammatory states, potentially delaying diagnosis. This case highlights the importance of systematic re-evaluation when clinical and biochemical findings diverge from a presumed single-organ pathology. Early multidisciplinary involvement and targeted therapy can be lifesaving in AOSD-HLH.