OBJECTIVE To study the effect of moderate altitude on thyroid hormones among training athletes. METHODS Serum total and free triiodothyronine (T3), thyroxine (T4), and thyrotropin (TSH) concentrations were measured among eight training athletes at sea level and subsequently during training for 12 days at the altitude of 1100-2700 m. METHODS Deaconess Institute of Oulu, and Division of Sports Medicine, Department of Physiology, University of Oulu, Finland. METHODS National top class skiers who normally live at sea level. RESULTS Serum concentrations of TSH, total and free T4, and reverse T3 did not change significantly. A significant decrease in serum total T3 levels from 1.9 nmol*1-1 on days 1-3 to 1.6-1.7 nmol*l-1 on days and 5-6 (p < 0.05) was observed. Serum albumin decreased (p < 0.05) on days 1-8 at altitude and correlated to serum total T3 (R = 0.36, p = 0.013). Serum free T3 increased steadily at moderate altitude and reached a significantly higher level (p < 0.05) at the end of the training period than before. CONCLUSIONS The serum free T3 increases when unacclimatized sea level skiers move to train at moderate altitude.