The purpose of the present study was to determine if serotonin alters human thyroid function. Fifteen patients with carcinoid tumors underwent clinical and laboratory evaluation of thyroid function. Ten of the subjects had excessive serotonin levels with the carcinoid syndrome; five of the subjects had normal serotonin levels. Five subjects also underwent monthly reevaluation for from five to 17 months after the initial evaluation. On initial evaluation 47 per cent of the patients had an abnormality of at least one of four tests of thyroid function (serum thyroxine, serum triiodothyronine, triiodothyronine resin uptake, or free thyroxine index). These abnormalities were found in patients with and without the carcinoid syndrome. Despite these abnormal laboratory tests, all of the patients were clinically euthyroid, and none had goiters. Although there is a high incidence of abnormal thyroid function tests in patients with carcinoid tumors, these alterations can not be attributed to serotonin.