Coexistence of Adult-Onset Still's Disease and Graves' Disease: Coincidence or Continuum?

Adult-onset Still's disease (AOSD) is a rare systemic autoimmune disorder of unclear etiology, typically characterized by prolonged fever, arthralgia, transient rash, and leukocytosis. Graves' disease (GD) is a common autoimmune cause of hyperthyroidism in young adults. Although associations between autoimmune conditions have been reported, the coexistence of AOSD and GD remains uncommon. We describe the case of a 22-year-old patient presenting with clinical and biochemical hyperthyroidism, confirmed by thyroid scintigraphy. The clinical picture was further complicated by persistent inflammatory syndrome, marked hyperferritinemia, and hepatosplenomegaly, consistent with AOSD as defined by Yamaguchi's criteria. The disease course was characterized by recurrent inflammatory flares of both conditions and the development of carbimazole-induced hepatocellular injury, necessitating discontinuation of the drug and subsequent radioiodine ablation. The patient was also treated with corticosteroid therapy, leading to favorable clinical and laboratory improvement. Several reports have documented the association between AOSD and GD, suggesting a potential shared pathophysiological basis. Both conditions exhibit genetic susceptibility linked to HLA-DRB1 alleles and share immune dysregulation driven by excessive cytokine production, particularly interleukin-18, tumor necrosis factor-α, and interferon-γ. This molecular overlap may account for their coexistence in some patients, regardless of sex. Systematic screening for thyroid dysfunction is advisable in patients with AOSD, irrespective of gender, in order to better identify this underrecognized association.