We measured the levels of serum folate and vitamin B12 in newly discovered hypothyroid (n = 56) and hyperthyroid (n = 47) patients and in age- and sex-matched control subjects (n = 103). Except for one patient with latent pernicious anemia, serum folate and vitamin B12 levels did not differ greatly in our patients and in our control subjects. Another patient was receiving monthly injections of cyanocobalamin for previously diagnosed pernicious anemia. We conclude that abnormalities of thyroid function per se did not alter serum folate or vitamin B12 levels in our patients.
Caplan RH, Davis K, Bengston B, Smith MJ. Serum Folate and Vitamin B12 Levels in Hypothyroid and Hyperthyroid Patients. Arch Intern Med. 1975;135(5):701-704. doi:10.1001/archinte.1975.00330050075013