A Rare Case of Anti-Yo Positive Paraneoplastic Limbic Encephalitis Associated with Thyroid-Parathyroid Adenoma.

Paraneoplastic neurological syndromes (PNS) are typically associated with malignant neoplasms. The anti-Yo antibody is classically linked to paraneoplastic cerebellar degeneration; its coexistence with limbic encephalitis is extremely rare. We report the first case of anti-Yo–positive limbic encephalitis associated with a benign thyroid–parathyroid adenoma. A 59-year-old male presented with epileptic seizures, left upper limb weakness, dysarthria, and confusion. Serum anti-Yo antibody was positive. Cranial MRI revealed diffusion restriction in the cerebellar hemispheres and middle cerebellar peduncles. PET-CT showed intense FDG uptake in the posterior right thyroid lobe. A fine-needle aspiration biopsy showed solid and cystic elements consistent with a parathyroid adenoma. The patient received high-dose intravenous methylprednisolone followed by intravenous immunoglobulin, resulting in clinical improvement. This report presents the first documented association between anti-Yo–positive limbic encephalitis and a benign thyroid–parathyroid adenoma. It emphasizes the importance of early immunotherapy and multidisciplinary evaluation in rare paraneoplastic syndromes.