Case of painless thyroiditis that developed during adjuvant chemotherapy for breast cancer

We report a case of painless thyroiditis that occurred during adjuvant chemotherapy for breast cancer. A 41-year-old woman was diagnosed with right breast cancer and underwent surgery followed by adjuvant chemotherapy, given the tumor size and relatively high ratio of Ki-67-positive cells. Three weeks after the first intravenous administration of fluorouracil (500 mg/m²), epirubicin (100 mg/m²), cyclophosphamide (500 mg/m²), and dexamethasone (6.6 mg/body), followed by 3 days of oral dexamethasone (8 mg/day), she complained of continued palpitations. Although Graves' disease was initially suspected, she was diagnosed with painless thyroiditis because of a low free T3:free T4 ratio and low thyroid uptake of iodine. The mechanism of painless thyroiditis, in this case, remains unclear, although supposed etiologic event was the use of cytotoxic drugs (including fluorouracil) or the withdrawal of short-term steroid administration. Painless thyroiditis very rarely occurs during chemotherapy. However, we should consider painless thyroiditis when a patient undergoing cytotoxic chemotherapy for breast cancer experiences continued palpitations. The appropriate diagnosis and treatment of symptoms might avoid an unnecessary dose reduction or discontinuation of chemotherapy and, moreover, may prevent adverse effects associated with the metabolism of anticancer agents.