Two clinically euthyroid patients with multinodular goiter were found to have high "free" thyroxin (Amerlex-M, Amersham and Coat-a-Count, DPC) and triiodothyronine (Amerlex-M) concentrations (FT4 and FT3, respectively). The presence of antibodies to T4 and T3 was confirmed by the finding that polyethylene glycol precipitated a far greater proportion of radioactivity when radiolabeled FT4 or FT3 analog (Amerlex-M) was incubated with serum from these patients than was true for normal subjects. With this method we could not demonstrate antibodies to thyroid hormones in 116 healthy volunteers. Of 101 hyperthyroid patients tested, one had antibodies to T4 but none had antibodies to T3. One patient had antibodies to T4, and one to T3, of 36 hypothyroid patients tested. All patients with thyroid hormone antibodies also demonstrated antithyroglobulin antibodies (measured immunoradiometrically). Evidently, the presence of thyroid hormone antibodies should be suspected when results of thyroid-function tests are discordant with the clinical state, and we suggest that measurement of thyrotropin by an assay with improved detection limits will aid in correctly determining thyroid status.