| Summary: | ABSTRACT: Objective: To describe a case of Graves disease (GD) and coexistent pancytopenia associated with autoimmune vitamin B12 deficiency. While thyrotoxicosis and antithyroid drugs can cause pancytopenia, other autoimmune conditions such as vitamin B12 deficiency can occur, leading to severe anemia and pancytopenia. Methods: A 19-year-old female with GD treated with methimazole presented with thyrotoxicosis and evidence of pancytopenia. Diagnostic studies included a complete blood cell count, peripheral blood smears, thyroid function tests, and a bone marrow biopsy. Results: White blood cells were 2.4 × 109 cells/L (reference range [RR] is 3.4 to 9.6 × 109 cells/L), hemoglobin was 7.9 g/dL (RR is 11.6 to 15.0 g/dL), neutrophil count was 1.2 × 109 cells/L, and platelets were 84 × 109 cells/L (RR is 157 to 371 × 109 cells/L). Thyroid-stimulating hormone was <0.01 mIU/L (RR is 0.50 to 4.30 mIU/L), free thyroxine was 3.7 ng/dL (RR is 1.0 to 1.6 ng/dL), and total triiodothyronine was 221 ng/dL (RR is 91 to 218 ng/dL). Due to suspicion for drug-induced pancytopenia, methimazole was discontinued. Three days later, she was hospitalized for a syncopal episode with a further decline in hemoglobin to 6.7 g/dL, neutrophils to 0.68 × 109 cells/L, and platelets to 69 × 109 cells/L. Bone marrow biopsy findings showing marrow hypercellularity and hypersegmented neutrophils suggested vitamin B12 deficiency. Vitamin B12 was <70 ng/L (RR is 180 to 914 ng/L). Intramuscular vitamin B12 injections were initiated, and pancytopenia resolved within 1 month. Conclusion: Although rarely described in the literature, autoimmune vitamin B12 deficiency can be missed as an underlying etiology for pancytopenia in patients with GD. The clinical picture can be further confounded when these patients are treated with antithyroid drugs known to cause bone marrow suppression. Abbreviations: ATD antithyroid drug GD Graves disease RR reference range WBC white blood cell
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