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Trastuzumab and thyroid dysfunction: An association to be aware of

The incidence of autoimmune thyroid disorders is higher among women with breast cancer (BC) than in other solid malignancies, while it has not a prognostic impact. Trastuzumab (T) is a humanized monoclonal antibody approved for human epidermal growth factor receptor 2 (HER2)-positive BC in the neoadjuvant, adjuvant, and metastatic scenarios. Since 2014, subcutaneous (SC) T has been employed with the same efficacy as the intravenous formulation together with an easier way of administration. To date, autoimmune thyroiditis has been linked rarely to the use of intravenous T, and no cases have been related to the SC presentation. We report two cases of HER2-positive early BC patients who developed hypothyroidism during maintenance therapy with SC T that required levothyroxine supplementation. SC T includes recombinant human hyaluronidase to facilitate tissue penetration of the drug. This enzyme may alter the thyroid gland stroma and facilitate the development of thyroid disorders. Thyroid function tests are recommended in patients on SC T.