What caused the macrocytosis?

Not iron deficiency. If that had been the case, the MCV would have dropped below the previous reading of 72 µm3 rather than increasing. Hence, restarting ferrous sulfate supplementation was-once again-inappropriate. Folate deficiency was a possible cause, as was alcohol, but the latter had been excluded, at least by history.

When the patient reported cold intolerance, her physician realized that the fatigue was not caused by anemia but was a manifestation of hypothyroidism. For unknown reasons, the MCV always climbs slightly (e.g., from 88 to 95 µm3) in anemia of hypothyroidism.
Hypothyroidism is a recognized complication of pernicious anemia. The condition develops in 5% to 10% of patients with pernicious anemia; hyperthyroidism develops in a smaller percentage. This is another example of why it is important to diagnose the underlying cause of vitamin B12 deficiency. I evaluate thyroid function annually in all patients with pernicious anemia.

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