A 27-year-old woman developed Wemicke's encephalopathy in the 16th week of her first pregnaney. She had thyroid storm and abnormal liver function. Her thyrotoxic symptom and abncemal liver function was recovered after medication of antithyroid drug, steroid, hepatotonic drug and administration of thiamine(fursulthiamin), but the fetus was lost, Thereafter her thyroid function returned to normal and euthyroid state was maintained without medication of antithyroid drug, but her neurological defect was remained. We suggest that severe hyperemesis gravidarum is a possible risk factor of the thyroid storm and Wemicke's encephalopathy in patients with hyperthyroidism, and consider the check of the thyroid function. The need for parenteral thiamine supplementation and medication of antithyroid drug is warranted in patients with severe hyperemesis gravidarum which lasts longer than 3 weeks and abnormal thyroid function.