[A case of phenytoin intoxication induced by hypothyroidism]

A 42-year-old woman who presented phenytoin intoxication induced by acute hypothyroidism was reported. She had a 29-year history of hypothyroidism and a 18-year history of epilepsy. She was treated with phenytoin (PHT) 100 mg, mephobarbital (MPB) 200 mg, valproic acid (VPA) 400 mg and thyroid powder 100 mg daily for 2 years. She had no medical problem until she noticed gait disturbance and diplopia which appeared 1 month after sudden withdrawal of thyroid powder. On admission, she was somnolent and somewhat disoriented. She had nystagmus in horizontal direction of gaze. Her speech was slurred and she could not sit nor stand due to trunkal ataxia. There was prominent intentional tremor in finger-nose test and heel-shin test showed severe ataxia. Blood cell count and blood chemistry examinations were normal. Serum PHT, phenobarbital (PB) levels were elevated as to 26.4, 36.4 micrograms/ml, respectively. VPA level was low. The endocrinological examinations revealed primary hypothyroidism. EEG showed generalized slow background, but cranial MRI, EMG, SEP and ECG were normal. Thyroxine (T4) administration was started soon, and in the course of thyroid hormone replacement, her cerebellar symptoms gradually improved and serum PHT level decreased even to the subtherapeutical level with the same amount of antiepileptic drugs treatment. By the 40th day of admission, thyroid function became normal and cerebellar signs disappeared, however, she needed 200 mg PHT daily to obtain good control of epilepsy. Cerebellar symptoms of this patient were thought to be PHT intoxication rather than ataxia caused by hypothyroidism itself from the viewpoint of clinical manifestations.(ABSTRACT TRUNCATED AT 250 WORDS)