The seasonal variations of iodothyronines and TSH serum concentrations were evaluated in 24 healthy subjects of both sexes. Using a 12 month cosine function, a significant circannual rhythm was found in T3, T4 and free T4 whose maximal values were found in September for T3, in August for T4 and in October for free T4. No significant seasonal variations in free T3 or TSH were found. Some circannual but not sinusoidal changes were found for reverse T3 and TBG. A positive linear correlation between T4, T3 and reverse T3 changes was observed, indicating that the sinusoidal oscillations of these hormones were in phase with each other. Relative weight was significantly lower in summer than in winter, though spontaneous caloric intake and physical activity did not change. A slight but significant correlation was found between relative weight and T4, free T4 and reverse T3 values measured during the period of study.