SAT-463 Thyrotoxicosis from Nivolumab in a Patient with Preexisting Graves’ Disease

Introduction Thyroid dysfunction is one of the common immune-related adverse events associated with immune checkpoint inhibitors like Nivolumab. Thyroiditis or primary hypothyroidism is the most commonly reported presentation. Graves’ disease is less frequently reported. We report a case of preexisting Graves’ disease patient, on antithyroid meds who developed thyrotoxicosis followed by hypothyroidism after receiving Nivolumab therapy. Case 66 y/o female patient with newly diagnosed metastatic melanoma presented to us for evaluation of abnormal thyroid test after her second cycle of Nivolumab. She has a long-standing history of Graves’ disease and has been on methimazole since her diagnosis. Her baseline thyroid labs before the start of Nivolumab were within normal limits (on methimazole 2.5 mg daily). She presented with weight loss, palpitations, and tremors four weeks after the start of Nivolumab. On exam, she was tachycardic with tremors noted to outstretched hands and had diffusely enlarged thyroid. Repeat lab work done before her second cycle revealed suppressed TSH 0.02 (0.4-4.5 uIU/ml) with elevated free T4 and T3. Her TSI titers were elevated. Methimazole dose was increased to 10 mg daily, and follow up labs done in a month revealed TSH of 89 uIU/ml, Free T4 0.16 (0.76-1.8 ng/dl). Methimazole was completely stopped at this time. She continued to have elevated TSH despite being off of methimazole for more than a month, concerning for the development of hypothyroidism. She was started on levothyroxine, after which labs returned to normal. The patient continued on immunotherapy during this period. Discussion Immune checkpoint inhibitors have been increasingly used for cancer therapy. Endocrinopathies are the most common immune-related adverse events associated with the use of these agents, with thyroid dysfunction being more common. Our patient had well-controlled Graves’ disease and was on a stable dose of methimazole for years. She developed autoimmune thyroiditis four weeks after receiving immunotherapy and subsequently developed hypothyroidism. The literature search did not reveal cases of autoimmune thyroiditis in a patient with preexisting Graves’ disease. One study reported that the timeline for developing the thyrotoxic phase is five weeks, which is followed by the rapid development of either euthyroid or hypothyroid phase. Management during the thyrotoxic phase is usually beta-blockers. Current guidelines recommend checking thyroid function test before initiation of therapy and every two weeks after the diagnosis of thyrotoxicosis until they become euthyroid or hypothyroid. Our case illustrates that patients with preexisting Graves’ disease can develop thyroiditis after receiving immune checkpoint inhibitors, and hence, frequent monitoring with thyroid function tests is needed.