Ryo Hara, Toshiyuki Kakumoto, A SUZUKI, Satoshi Yamashita, Takayoshi Niwa, Kanako Hayashi, Arisa Morizono, Ayaka Sato, Masahiko Tanabe, Noritaka Kudo, Masashi Hamada, Wataru Satake, Tatsushi Toda
ABSTRACT Background Hashimoto's encephalopathy (HE) is a rare steroid‐responsive autoimmune syndrome. Although HE is associated with anti‐thyroid antibodies, its concurrence with malignant thyroid tumors has rarely been described and its clinical implications remain poorly understood. Case Presentation A 54‐year‐old euthyroid woman presented with a 5‐month history of progressive cognitive decline and behavioral changes, leading to profound hypolalia and restlessness. Serum anti‐thyroglobulin and anti‐thyroid peroxidase antibodies were markedly elevated. Brain MRI, electroencephalography, and cerebrospinal fluid tests were unremarkable. Corticosteroid therapy resulted in rapid resolution of neuropsychiatric symptoms and cognitive dysfunction. Subsequent thyroid evaluation identified papillary thyroid carcinoma (PTC) on a background of chronic lymphocytic thyroiditis. The patient underwent total thyroidectomy and radioiodine therapy. At 1‐year follow‐up, she remained free of HE relapses and cancer recurrence, with a corresponding decline in anti‐thyroid antibody titers. Conclusions This is the first reported case of HE occurring concurrently with PTC. The findings raise the possibility that a shared autoimmune milieu may predispose to both HE and PTC. This case underscores the importance of comprehensive thyroid evaluation, including oncological screening, in patients with HE.