BACKGROUND: Vitamin B12, also known as cobalamin (Cbl), is a major player in both erythropoiesis and myelination of the central nervous system. The 2 main manifestations of vitamin B12 deficiency are bone marrow failure and demyelinating disease. These manifestations also depend on the severity and duration of the deficiency. In severe cases, ineffective erythropoiesis, with intramedullary destruction of erythrocytes has been reported, and increased homocysteine level was shown to be a cause of this hemolysis, in vitro.

CASE REPORT: We present the case of a middle-aged man presented with worsening fatigue, pallor, and dyspnea on moderate exertion. He was found to have a macrocytic anemia associated with intravascular hemolysis and clinical hypothyroidism in the setting of Hashimoto’s thyroiditis. Vitamin B12 measured as part of his anemia investigation, was found to be markedly deficient (less than 100 pg/mL). Replacement was started and a reversal of his anemia and hemolysis was shown with improvement in his hemoglobin level upon follow-up.

CONCLUSIONS: Although a rare presentation, severe vitamin B12 deficiency can lead to hemolysis and severe anemia that can be life threatening. Timely and appropriate diagnosis and replacement, lifelong in some cases, in addition to looking for the underlying cause of this deficiency, and excluding other concomitant hemolytic disorders, is crucial for the management of this reversible disease. Despite some in vitro early results, the exact underlying mechanism behind hemolysis is still unclear.

Keywords: Anemia, Hemolytic, Anemia, Pernicious, homocysteine, Vitamin B 12 Deficiency, Diagnosis, Differential, Erythrocyte Transfusion, Hashimoto disease, thyroxine, Vitamin B 12