In two runs over a distance of 75 km and 45 km as well as in a marathon (42.2 km), the thyroid hormones (T4, T3 and TSH in all runs and rT3 in two of them) were determined before and after the runs. The performance of the runners was analyzed, with their age considered in the analysis. We found that the changes of thyroid hormones are characterized by individual differences. Younger runners and those with better performances had significantly higher T4 serum concentrations and unaltered T3 and TSH concentrations, whereas older sportsmen and those with worse performances reacted to the stressful stimulus with a significant decrease of T4, T3 and TSH. This response was interpreted as an exhaustion reaction, i.e. reduced adaptation responses of thyroid hormone metabolism. Therefore, these investigations provide an assessment of the state of fitness, the optimization of training and the avoidance of damage caused by overly stressful physical exercise