Aims: The aim of this investigation was to examine the thyroid hormone levels of professional cyclists during a 3-week stage competition (Vuelta a España 1998). Methods: The study population was made up of 16 male cyclists from two world-leading professional teams. Four blood samples were drawn (between 07:00 and 09:00 a.m.) from each participant before and at the end of the 1st, 2nd and 3rd weeks of competition. 3,5,3′(-Triiodothyronine (T3), free T3 (FT3), thyroxine (T4), free T4 (FT4) and thyroid-stimulating hormone (TSH) were determined in each blood sample by radioimmunoassay. Results: Serum T4, FT4 and FT3 levels showed a significant increase (p < 0.05) by the last week of competition while concentrations of TSH and T3 remained unchanged. Conclusion: In conclusion, 3 weeks of competition provokes changes in basal thyroid hormone concentrations in professional road cyclists.