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Overt Hypothyroidism Induced by Prolonged Therapy With Imatinib


 Background: Imatinib, a tyrosine kinase inhibitor (TKI), is commonly used to treat chronic myelogenous leukemia (CML) and gastrointestinal stromal tumors. TKI-induced thyroid dysfunction is recognized as an adverse class effect with most cases occurring between six to twelve months after treatment initiation.
 Clinical Case: 76-year-old man with hypertension and CML on Imatinib that had been started twenty months prior was admitted for confusion. The patient also reported constipation, cold intolerance, and weight gain despite no change in diet, appetite, or physical activity. On evaluation, his responses to questioning were noticeably delayed, however he was not lethargic. His vital signs were normal and he was otherwise euthyroid on physical exam. He had no known history of thyroid dysfunction. Labs showed a TSH of 94.7 (0.4 - 4.2 uIU/mL), Free T4 0.4 (0.8 - 1.5 ng/dL), and Total T3 30 (87 - 178 ng/dL). Thyroglobulin antibody was 1016 (0.0 - 4.1 U/mL) and TPO antibody was >2000 (0.0 - 5.6 IU/mL). A TSH checked two months before admission was 1.1. Antibody levels had not been checked previously. Thyroid ultrasound demonstrated a hyperemic and heterogeneous thyroid, consistent with thyroiditis. Levothyroxine at a dose of 50 mcg daily was advised, due to patient’s advanced age as well as history of arrhythmia. The patient’s confusion resolved on hospital day three. Repeat thyroid function tests will be checked four to six weeks after Levothyroxine initiation.
 Discussion: As a class, tyrosine kinase inhibitors are known to cause thyroid dysfunction with the most common medication being Sunitinib. Data related to the effects of thyroid function during Imatinib treatment are limited. Previous cases of Imatinib-induced thyroid dysfunction report only hypothyroidism in thyroidectomized patients and no clinically significant change in thyroid function among patients who were euthyroid prior to therapy initiation. Our case reports a patient with no prior thyroidectomy who developed overt hypothyroidism while on Imatinib for nearly two years. The mechanism for TKI-induced thyroid dysfunction has not been elucidated. Due to relatively acute onset and markedly positive TPO and thyroglobulin antibodies, we suspect that TKI may alter HLA recognition on thyroid follicular cells, thereby inducing autoimmunity. Our case showcases the need to maintain awareness and continuous surveillance for thyroid dysfunction when patients are on long term TKI as overt hypothyroidism may be induced by prolonged treatment.