7410 Bevacizumab Induced Hypothyroidism

Abstract

Abstract Disclosure: A.L. McKenna: None. S. Thota-Kammili: None. K. Grennan: None. S. Rao: None. It is known that thyroid dysfunction is an adverse effect of tyrosine kinase inhibitors (TKIs) via inhibition of the vascular endothelial growth factor receptor (VEGFR). The VEGF signaling pathway is important in regulating angiogenesis in tumors and normal tissue. As a highly vascularized gland, it is expected the thyroid may be compromised because of VEGFR inhibitors. Proposed mechanisms of thyroid dysfunction include capillary regression and constriction leading to reduced vascular perfusion to the gland. Bevacizumab is a monoclonal antibody that targets VEGF and inhibits activation of VEGFR, reducing angiogenesis; interestingly, this is not commonly reported in the literature. Here we present a case where bevacizumab altered thyroid function in a patient with pre-existing primary hypothyroidism. A 60-year-old woman presented with worsening hypothyroidism. She reported a history of nonautoimmune primary hypothyroidism diagnosed in her early 20s; she still had an intact thyroid with no history of head/neck radiation. In the months leading to the presentation, she had been receiving maintenance chemotherapy with 5 fluorouracil and bevacizumab every 2 weeks for metastatic duodenal carcinoma. Prior to starting chemotherapy, she had been on a steady dose of 112 mcg of levothyroxine (LT4) daily. After 6 months on chemotherapy, she reported symptoms of fatigue and hair loss. Workup revealed an elevated TSH of 41 mIU/L (reference 0.3-4.2 mIU/L). She denied any significant changes in weight or malabsorption issues. Aside from the chemotherapy, no other new medications were introduced. Her LT4 dose was increased to 137 mcg daily, and 3 months later, there was improvement in TSH to 6.5 mIU/L as well as her symptoms. At this point, bevacizumab was briefly on hold for 2 months due to an upcoming procedure, hence there was no change to LT4 to avoid over correction. Her TSH normalized and was trending lower while bevacizumab was on hold. Once bevacizumab resumed, LT4 was preemptively increased to 150 mcg daily. Four months after resuming bevacizumab, her TSH remains within the normal reference range on 150 mcg LT4. It is hypothesized that this VEGF blocker led to worsening of her underlying hypothyroidism.Given the shared down regulation in the VEGF signaling pathway, it would be expected that TKIs and monoclonal antibodies against VEGF would share similar adverse reactions to the thyroid. However, to our knowledge, there have been limited reports of bevacizumab induced thyroid dysfunction in the literature. Explanations to this may include the more targeted VEGF inhibition with bevacizumab versus the inhibition of multiple kinase receptors from the TKIs. Further research into this will be needed. This case demonstrates that bevacizumab can impact thyroid function and thyroid labs may need to be monitored at the onset and during VEGF inhibitor therapies. Presentation: 6/3/2024