Serum and red cell folate and serum vitamin B12 levels in hyperthyroidism

Serum and red blood cell folate levels and serum B12 concentration were determined by radioassay in 20 hyperthyroid patients and compared with values obtained when the same patients had been euthyroid for at least 4 months. In hyperthyroidism, the levels of serum and red blood cell folate were significantly (P less than .01) higher than when euthyroidism was achieved. There was no significant change in serum B12 concentration. Declines in serum and red blood cell folate levels between hyperthyroidism and euthyroidism occurred in 15 and 16 of the 20 patients, respectively. Although the explanation for the relative elevations of serum and red blood cell folate levels in hyperthyroid patients is unclear at present, our findings do not support the view that hyperthyroidism in man is associated with depletion of folate stores or subclinical deficiency of the vitamin.