Background: Steroid-responsive encephalopathy associated with autoimmune thyroiditis (SREAT) is an autoimmune disease that appears as a fulminant, subacute or chronic course of altered mental status, often accompanied by seizures and myoclonus. This case report demonstrates that SREAT can be present with solitary neuropsychiatric signs long before seizures and myoclonus. Methods: A 42-year-old female presented with abrupt-onset whole-body myoclonic jerks. She had been suffering from depression for 15 years before being diagnosed with hypothyroidism. She was conscious, oriented and alert. At rest, all four limbs had multifocal mild-to-moderate myoclonus, which was significantly aggravated by muscle activation; her Hamilton Depression Rating Scale score was 21. Results: Antithyroglobulin and antithyroid peroxidase autoantibodies were both above 2,000 IU/mL. The thyroid-stimulating hormone level was 5.65 mIU/mL, free triiodothyronine level was 3.36 pg/mL and free thyroxine level was 1.02 ng/dL. Vasculitis profile and the serum test for autoimmune encephalitis panel were negative. Brain neuroimaging was normal. Pulse dose of methylprednisolone followed by oral steroids resulted in significant clinical improvement. Conclusion: SREAT can present with chronic neuropsychiatric symptoms with abrupt exacerbation of seizures and myoclonus. This case study emphasizes the importance of screening individuals with depression and thyroid problems for serum antithyroid antibody levels. In most cases, steroid treatment yields positive results.