Unveiling the Pernicious Truth: A Case Report on the Rare Presentation of Severe Vitamin B12 Deficiency.

Vitamin B₁₂ deficiency typically causes megaloblastic anemia but can rarely present with hemolytic features, leading to diagnostic confusion with autoimmune or microangiopathic processes. Pernicious anemia, an autoimmune cause of severe B₁₂ deficiency, is frequently underrecognized and may coexist with other autoimmune disorders. We report a 45-year-old woman with Hashimoto's thyroiditis who presented with dizziness, profound fatigue, and symptomatic anemia. Laboratory evaluation revealed severe macrocytic anemia (hemoglobin 3.5 g/dL, mean corpuscular volume (MCV) 136.9 fL) with thrombocytopenia, elevated lactate dehydrogenase (LDH) (2905 U/L), indirect hyperbilirubinemia, and suppressed haptoglobin, findings suggestive of hemolysis. Peripheral smear showed macrocytosis and marked anisopoikilocytosis without schistocytes. Coombs testing was negative, and G6PD levels were normal. Vitamin B₁₂ was severely reduced (73 pg/mL), and anti-intrinsic factor antibodies were positive, confirming pernicious anemia presenting as hemolytic anemia. Intensive intramuscular cyanocobalamin therapy led to rapid hematologic recovery, including normalization of platelet count within two weeks and improvement of hemoglobin to 8.4 g/dL. Pernicious anemia can mimic hemolytic anemia, risking misdiagnosis as autoimmune hemolysis or thrombotic microangiopathy. Negative Coombs test, mild reticulocytosis relative to anemia severity, and macrocytosis provide critical diagnostic clues. Awareness of autoimmune clustering such as Hashimoto's thyroiditis with pernicious anemia can prompt timely recognition. Early vitamin B₁₂ replacement reverses hematologic abnormalities and prevents irreversible neurologic damage.