Previous reports demonstrate a circadian rhythm of the free thyroid hormones in healthy subjects. In this study we evaluated circadian variation of FT3 and FT4 in hyperthyroid and hypothyroid states. We considered six hyperthyroid patients, six hypothyroid patients and six control subjects. Blood samples were taken two hours apart from a catherterized arm vein. Data were evaluated by Halberg's cosinor analysis. The results show that FT3 and FT4 exhibit a circadian rhythm in healthy subjects, not evident in hyperthyroid and hypothyroid patients.