Autoimmune hypothyroidism and trastuzumab therapy: a rare association

Abstract

We report a case of a woman with a diagnosis of breast cancer who unintentionally started gaining weight, feeling tired, and constipated 44 weeks after the initiation of trastuzumab. Hypothyroidism secondary to an autoimmune thyroiditis associated with trastuzumab was diagnosed, the first case described in Portugal and the fourth case described worldwide. Our intention regarding the publication of this case report is to alert the clinicians treating people with trastuzumab that they should ask the patients about symptoms of hypothyroidism and should screen the thyroid function of the patients before, during, and after the initiation of trastuzumab.