Successful Management of Hyperthyroidism With Lithium and Radioiodine in a Patient With Previous Methimazole-Induced Agranulocytosis

Introduction: Antithyroid drug (ATD) therapy is the first-line treatment of Graves’ hyperthyroidism. Agranulocytosis, although rare, is a life-threatening condition associated with ATD therapy. For patients who recovered from ATD-induced agranulocytosis, surgery or radioiodine (RAI) therapy are adequate options to restore the patient’s euthyroid state. Here we report a case of ATD-related agranulocytosis where lithium therapy was used before RAI to control thyrotoxicosis and prevent worsening of hyperthyroidism. Case Report: A 74-year-old female with a previous history of hypertension presented with a 2-month history of weight loss (12 lbs), palpitations and shortness of breath. She was afebrile with a heart rate of 110, a blood pressure of 149/80, a fine tremor and a moderate diffuse goiter. She had a normal eye exam. Laboratory evaluation demonstrated TSH <0.01 uIU/mL (0.35-5.5), FT4 3.11 ng/dL (0.51-1.65) and TSH receptor antibody (TRAb) 40 (<1,0 U/L), consistent with thyrotoxicosis due to Graves’ disease. She was started on methimazole (MMI) 15mg and metoprolol. After four weeks, symptoms resolved and thyroid function tests (TFT) improved. However, after two months of treatment, she was hospitalized for fever, diarrhea and abdominal pain. White blood cell count (WBC) was 650/μL, and neutrophil count was 90/μL. A diagnosis of gastroenteritis and agranulocytosis was made and MMI was stopped. After seven days, symptoms resolved, the neutrophil count was 2200/ul and TFT were acceptable (FT4 1.25, ng/dL TT3 1.67 ng/ml, TSH < 0.02 uIU/mL). She was discharged without ATDs and a RAI dose of 20 mCi was scheduled. However, RAI therapy had to be postponed due to COVID-19 pandemic restrictions. After 3 weeks, TFT worsened and therapy with lithium carbonate 300 mg TID was started as the patient refused thyroidectomy. Lithium was initiated 12 days before RAI therapy and was maintained 7 days after the procedure. No side effects associated with lithium treatment were reported. TFT 7 days after RAI were FT4 1.43, ng/dL TT3 2.05 ng/ml and TSH < 0.02 uIU/mL. One month later, the patient was euthyroid without need for thyroid medication and remains on follow up. Discussion: Serum thyroid hormone (TH) concentrations usually increase after RAI therapy for Graves’ disease, a worrisome fact in patients with increased risk for cardiovascular complications. Previous studies report that pre-RAI treatment with lithium prevents changes in serum TH concentrations and enhances RAI therapy’s effectiveness. Here, treatment with lithium was used to control thyrotoxicosis and prevent further increase in TH levels associated with RAI therapy. Lithium is particularly suitable for patients with ATD-related side effects before definitive therapy (radioiodine or thyroidectomy). The antithyroid effect of lithium in this setting should be further studied.