The response to small doses of vitamin B12 was studied in a woman with pernicious anemia and thyrotoxicosis. No response to 1 µg. per day occurred, but she did respond to 10 µg. per day. Fourteen months later when she was euthyroid, the patient did respond to 1 µg. per day. In another study, serum vitamin B12 levels were significantly lower in a group of patients with hyperthyroidism than in a group of normal subjects. Seventeen of 20 patients with hyperthyroidism exhibited significantly higher serum vitamin B12 levels 5-8 months after becoming euthyroid. The results of these studies provide evidence for increased vitamin B12 requirements in the presence of hyperthyroidism. The greater need for vitamin B12 appears to be related to increased utilization and/or accelerated turnover imposed by increased metabolism.