| Summary: | Objective: Tyrosine-kinase inhibitors (TKIs) are chemotherapeutic agents associated with increased thyroid-hormone requirements and altered deiodinase activity. We present the first case to link these findings to the TKI ibrutinib. Methods: Serial thyroid-stimulating hormone (TSH), free-thyroxine (FT4), free-triiodothyronine (FT3), and reverse-triiodothyronine (rT3) levels were assessed. Results: An 80-year-old, 62-kg woman with hypothyroidism secondary to total thyroidectomy for stage I papillary thyroid cancer, on maintenance levothyroxine (LT4) 137 μg daily, presented for follow-up. Compared to one year prior, the patient’s weight had increased by 2 kg and TSH from 2.58 to 27.60 μIU/mL (normal: 0.45-4.50 μIU/mL) while on pantoprazole. Ibrutinib, her other medication, had been started seven months prior for chronic lymphocytic leukemia. Despite sequential confirmation of proper LT4 adherence and self-administration, adjustment of LT4 to 150 μg, and discontinuation of pantoprazole, the patient’s hypothyroid symptoms worsened, and the TSH was 73.90 μIU/mL six months later. LT4 was increased to 175 μg six days a week and 262.5 μg once weekly. Two months later, the TSH was 3.92 μIU/mL (steady-state condition), FT4 2.32 ng/dL (normal: 0.82-1.77 ng/dL), FT3 1.6 pg/mL (normal: 2.0-4.4 pg/mL), and rT3 69.6 ng/dL (normal: 9.2-24.1 ng/dL). Ibrutinib was discontinued the next month due to gastrointestinal side effects and elevated blood pressure. Four months later, LT4 had been reduced to 150 μg, and the FT4 reached 1.92 ng/dL, FT3 2.0 pg/mL, and rT3 26.6 ng/dL. Conclusion: This report links ibrutinib to increased thyroid-hormone requirements in a thyroidectomized woman whose decreased T3:T4, T3:rT3, and T4:rT3 ratios suggested type 3 deiodinase induction and type 2 deiodinase inhibition.
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