We present a case of a patient who was diagnosed with Hashimoto's encephalopathy based on the presence of subacute behavioural changes, negative work up for infection and immunological serology except for high serum titres of thyroid autoantibodies. Thyroid function tests (TFTs) and MRI of the brain were normal. EEG showed low amplitude, slow waves and θ waves at both frontal areas. His condition improved dramatically after treated with high-dose glucocorticoid. After 2 years of a relapsing-remitting course, a new episode occurred. There was an abrupt change of TFTs within 5 days: free thyroxine (fT4) from 1.52 to 1.53 ng/mL, free triiodothyronine (fT3) from 3.25 to >30 pg/mL and thyroid-stimulating hormone (TSH) from 5.08 to 0.78 mIU/L. On the following day found fT4 2.58, fT3 14.67 and TSH 0.042. The patient was diagnosed with Hashitoxicosis. High-dose glucocorticoid and ß-blockers were initiated. The symptoms gradually improved and TFTs normalised within 2 weeks.