The aim of the research was to study the total and free iodothyronine (T3, T4, fT3, fT4) changes of trained horses transported before and/or after competition, and performing in show jumping competition, respectively. Six horses were studied in four different sessions during four consecutive weeks. During the first week, horses were submitted to transport before competition (sessions: t0 = before transport, T0 = after transport). Then, during the second week, horses were submitted to transport after competition (sessions: t1 = before transport, T1 = after transport). During the third week, horses participated in a competitive show jumping event (sessions: t2 = before competition, T2 = after competition). During the fourth week, horses were submitted to transport and to a competitive show jumping event (sessions: t3 = after transport and before exercise, T3 = after exercise). Compared with t0 session, t1 showed higher T3 (P < .01) and lower fT4 (P < .01) concentrations. Compared with t2 session, t3 showed higher T3 (P < .01) and lower fT3 (P < .01) concentrations. A significant effect of competition on fT3 changes (F = 4.227; P < .05) was recorded. The results obtained showed that transport and show jumping competition differently influenced the thyroid responses of trained sport horses to physiological and psychological stress, also according to the individual experimental sessions.