Autoimmune hypothyroidism and trastuzumab therapy: a rare association.

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.