Summary: 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.
Learning points: Trastuzumab is a humanized MAB used in HER2-positive breast and gastric cancer. Trastuzumab-associated autoimmune thyroid disease (AITD) is rare (incidence rate in an RCT of 0.3%). Manifestations of autoimmune thyroiditis associated with trastuzumab resemble those of hypothyroidism in other clinical contexts, but the presence of goiter is highlighted as a reason for medical evaluation. Biochemically, it is characterized by an increased thyroid-stimulating hormone (TSH) with or without a low FT4/FT3, and sonographically with a pattern of thyroiditis. The treatment consists of levothyroxine, in a dose of 1.6-1.8 µg/kg/day, with re-evaluation of the thyroid function in 4-6 weeks. We report the first case of autoimmune thyroiditis secondary to trastuzumab in Portugal. It is important to evaluate the thyroid function before, during, and after the initiation of this therapeutic agent.