The effect of exercise on circulating T3, T4, free (f) T3, and fT4 concentrations were studied before and after competitive show jumping in 24 trained jumper horses with an age range of 11–13 years and having the same level of show experience. All horses, randomly divided into three groups paired by gender, participated in three competition levels with the same circuit design over 10 fences, with five upright and five cross-pole fences. The three fence height groups were group I (n = 8), 1.10 m; group II (n = 8), 1.20 m; and group III (n = 8), 1.30 m. Repeated measures analysis of variance showed a statistically significant effect of competitive exercise on fT3 changes in all groups, whereas the effect of competitive exercise on T3 changes was exclusively seen in group III. Two-way repeated measures analysis of variance showed that the effect of fence height (P = .0002) and performance results were significant (P = .0486) only for T3 changes. Performance also affected fT3 changes (P = .0152). No significant correlation was found between total and free iodothyronines. These results suggest a dependence of T3 and fT3 changes on exercise-related variables and performance results and that the thyroid response is different with respect to the different exercise-related variables, such as the varying level of difficulty and performance results.