SAT-325 - Management of Graves' Disease with Methimazole in Patients with Duffy-Null Phenotype

Backgrounds:
Red blood cell antigen typing facilitates the diagnosis or exclusion of Duffy-null associated neutrophil count (DANC). Many people of African ancestry have lower absolute neutrophil counts (ANCs) without increased risk for infection. This is associated with the Duffy-null phenotype (non expression of the Duffy antigen on red blood cells), which is commonly found in those of African descent. The guidelines note that those of African American descent are frequently found to have low WBC counts, herein alluding to the Duffy-null phenotype, and that ANC values between 1 and 2 K/μL should not preclude treatment initiation. While no discrete evidence exists to correlate preexisting neutropenia to antithyroid drugs (ATD) related complications, the American Thyroid Association does recommend reconsideration of ATD use among those with moderate-severe neutropenia (ANC < 1 K/μL) out of caution for drug-induced agranulocytosis. This case series describes African American patients with DANC and ANC < 1000/μL who were successfully treated with methimazole.

Case description:
Two African American patients with newly diagnosed Graves’ disease and persistent neutropenia (ANC < 1000/μL) were initiated on methimazole. Baseline thyroid function tests and thyrotropin receptor antibody (TRAb) titers were obtained. Patients were monitored with serial CBCs, thyroid function, TRAb and clinical assessments.
Outcomes:
• Case 1: A 39- year-old woman with ANC 790/μL, TSH < 0.01 mIU/L, free T4 3.2 ng/dL, TRAb 12.5 IU/L. She was treated with methimazole 10 mg daily. The ANC remained stable and thyroid function normalized within 12 weeks.
• Case 2: A 42-year-old man with ANC 860/μL, TSH < 0.01 mIU/L, free T4 2.9 ng/dL, TRAb 9.8 IU/L. He was treated with methimazole 15 mg daily. The ANC remained stable, thyroid function normalized, and TRAb levels declined. Neither patient developed agranulocytosis or infectious complications.

Conclusion:
Some guidelines advise avoiding methimazole when baseline ANC is < 1000/μL, but strict adherence to this cutoff may inadvertently worsen health inequities by limiting ATD use in African American patients with DANC. Recognizing DANC as a benign physiologic variant enables individualized risk–benefit assessment and safe methimazole initiation with ATD–induced agranulocytosis; however, misinterpretation of this phenotype can lead to unnecessary treatment interruption and disparities in care. Standardized counseling on ATD adverse effects and prompt evaluation of warning symptoms remains essential for all patients.

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