Thyroid hormone antibodies (THAbs)--i.e., antibodies to thyroxin (T4) and triiodothyronine (T3)--are detected rarely in human serum, where they are searched for, possibly because of a quantitatively minimal interaction between thyroid hormones (the haptens) and serum IgGs (the antibodies). The weak binding could result from these facts: (a) there are already six physiological carrier proteins for thyroid hormones; (b) THAbs usually account for a very small fraction of the total serum IgGs; (c) THAbs may have--as reported in the literature--a relatively low affinity. To ascertain whether THAbs could pass undetected in serum, we measured antibodies to T3 and T4 in both the serum and the corresponding IgG fraction of six normal persons and 45 patients with various thyroid diseases (Graves' disease, idiopathic myxedema, Hashimoto's thyroiditis, subacute thyroiditis, tumors), using radioimmunoprecipitation. The prevalence of antibodies to T4 was 0/51 in both the sera and the IgG fractions; the prevalence of antibodies to T3 was 1/51 in both materials. Because all of the sera that tested THAb negative were confirmed to be so in the THAb assay of the IgG fraction, we conclude that the prevalence of serum THAbs is not underestimated and that autoimmunization against thyroid hormones is really a rare phenomenon.