Stiff-person syndrome with concurrent Graves disease: a rare autoimmune overlap.

Stiff-person syndrome (SPS) is an autoimmune neurologic disorder characterized by progressive rigidity and spasms, often linked to antiglutamic acid decarboxylase 65 antibodies. While SPS has established associations with autoimmune diseases like type 1 diabetes and celiac disease, co-occurrence with hyperthyroidism, particularly Graves disease (GD), is rare. We present the case of a 34-year-old woman with paranoia, hallucinations, and altered mental status. Labs revealed thyrotoxicosis, with positive thyroid-stimulating immunoglobulin and thyrotropin receptor antibodies, confirming GD. Despite antithyroid treatment, her neuropsychiatric symptoms progressed. Though the neurologic workup was unremarkable, cerebrospinal fluid studies revealed oligoclonal bands with markedly elevated antiglutamic acid decarboxylase 65 antibodies, consistent with autoimmune encephalitis within the SPS spectrum. Therapeutic plasma exchange and rituximab resulted in rapid clinical improvement. At 6 weeks, thyroid-stimulating immunoglobulin and thyrotropin receptor antibody levels normalized, and she remained stable off antithyroid therapy, suggesting resolution of GD. This case illustrates the rare coexistence of SPS and GD.