Lithium, prescribed for bipolar disorder, is known to induce thyroid dysfunction, most commonly hypothyroidism. Thyrotoxicosis due to lithium-induced painless thyroiditis is a rare complication. We have previously reported that the serum concentration of immunosuppressive acidic protein (IAP), an α1-acid glycoprotein, increased during the acute phase of subacute thyroiditis, but was within the normal range in patients with painless thyroiditis. In the present case, a 31-year-old woman, receiving long-term lithium therapy for bipolar disorder, had a recurrent episode of painless thyroiditis, and her serum IAP was increased. The pathogenic mechanism for lithium-associated painless thyroiditis may differ from that of autoimmune conditions.