Hashimoto's encephalopathy is a rare association of Hashimoto's thyroiditis. The clinical course is often characterised by relapses.1-2 Persistent fever has not been previously recorded in the literature.

A 48 year old white woman was referred in April 1996 from a psychiatric clinic, where she had presented with psychotic episodes with gradual deterioration and been diagnosed as possibly having Creutzfeldt-Jakob disease. She was confused and had brisk tendon reflexes, an extensor left plantar response, bilateral moderate cogwheeling, action myoclonus, and nuchal rigidity. The gait was unstable and uncoordinated.

Puerperal pcychosis occurred at the age of 22 and from the age of 30 she had periodic mild confusional states of about a week long. Hashimoto's thyroiditis was diagnosed 10 years before the neurological admission and since then the patient had been euthyroid on thyroxine replacement, although confusional states appeared periodically, roughly every 3–4 months.

After admission the clinical features changed, with variable stupor, generalised seizures and myoclonus, incontinence, and anorexia. There was a persistent fever which varied between 37.7 and 39^oC, with little or no responce to antipyretic drugs. Antiepileptic drugs achieved some control of the seizures.

Normal or negative laboratory tests included blood count, CSF analysis, erythrocyte sedimentation rate, liver and kidney function tests, coagulation screen, serum and CSF electrophoresis, immunoelectrophoresis, ANCA, anti-DNA, ASMA, antimitochondrial, antiphospholipid, and antinuclear antibodies, C reactive protein, tumour markers, serum copper, ceruloplasmin, serum electrolytes, vitamin B12 and folic acid, serological tests for HIV, HSV, HBV, CMV, syphilis, Widal, Wright, and PPD. Blood, CSF, and urine cultures were negative. The EEG showed diffuse slow activity. Brain CT and MRI were normal. T3, T4, TSH, FT3, and FT4, were normal but the titre of autoantibodies to thyroid peroxidase (Anti-TPO) was increased at 7000 mIU\l (normal value<60 mIU\l).

Treatment with prednisolone (70 mg\day) led to gradual improvement and the fever ceased promptly. Within 2 months the patient had fully recovered with only a residual external (left) plantar response. Decrease of the Anti-TPO (1000 mIU\l) titre paralleled the clinical improvement. The patient was discharged after 4 months having made a good recovery on 40 mg\day prednisolone and had no relapses during 3 subsequent years (maintenance prednisolone dose 15 mg every other day, Anti-TPO titre within normal range).

The clinical features in this case are compatible with a diagnosis of Hashimoto's encephalopathy with an associated fever. Both fever and Anti-TPO concentrations were supressed with prednisolone treatment. Increase of TNF-α concentration is seen in the serum of patients with autoimmune thyroiditis3 and this is known to act as an endogenous pyrogen.4 This mechanism may explain the persistent fever as a prominent symptom in our case .