Hypothyroidism causing macrocytic anemia unresponsive to B12 and folate.

A patient with pernicious anemia who developed a mild macrocytic anemia after many years of monthly vitamin B(12) therapy is presented. There was no response to the addition of folic acid and multivitamins. Evaluation for causes of elevated mean corpuscular volume and consideration of associated autoimmune disorders led to the diagnosis of primary hypothyroidism. Treatment with levothyroxine led to correction of the hematologic abnormalities over a four-month period. The anemia of hypothyroidism may present with macrocytic indices and this diagnosis should be suspected when this finding is noted.