Reasons for performing study: Involvement of thyroid function on performance warrants further investigation as limited data exists on the effects of showjumping on the dynamics of total and free iodothyronines.
Objectives: To investigate the response of circulating total and free iodothyronines in horses to experimental showjumping sessions and compare with the effects normally induced by competition and determine if fence height has any effect.
Materials and methods: Using a randomised crossover study design 6 trained horses were studied during experimental showjumping sessions over 10 fences of different height: 1.00 m (Session 1), 1.10 m (Session 2), 1.20 m (Session 3). Hormone levels were recorded before, after warm-up, 5 and 30 min post exercise. T3, T4, fT3, fT4 concentrations were analysed by ELISA/competition using streptavidin technology. RM-ANOVA was applied to test for any differences in basal and warm-up values of different sessions. Two way RM-ANOVA was applied to test for any effects of interaction between fence height and time. The differences between individual means over time were assessed using a post hoc multiple comparison test (Bonferroni).
Results: Basal T4 changes over the sessions (P<0.05) were recorded. After warm-up, T4 concentration results were lower than basal in Session 1 (P<0.05). Higher than basal values were recorded 30 min post exercise for T3 (P<0.001), T4 and fT4 (P<0.01) in Session 2 and for T4 (P<0.05) and fT4 (P<0.01) in Session 3. The interaction fence height/time results were significant on T3 (P<0.03) and fT4 (P<0.03); sampling time on T3 (P<0.0007), T4 (P<0.001) and fT4 (P<0.002) post exercise changes.
Conclusion: Showjumping over the highest fences induced a release of T3 from skeletal muscle, probably due to 5′-desiodase activity and increase of fT4, due to thyroid stimulation and/or changes in capacity to bind iodothyronines.