Effect of increasing exercise levels, feeding-housing regimes and sex on the total and free iodothyronines of trained horses

The aim of the present study was to assess and compare the effect of increasing exercise levels of thirty-six trained Dutch Warmblood horses fed a isoenergetic high versus low fiber content on the circulating Thyroid hormones (THs): triiodothyronine (T3), thyroxine (T4), free triiodothyronine (fT3) and free thyroxine (fT4) concentrations. The training schedule was carried out in three consecutive phases, according to three exercise levels (light, medium and medium-heavy) performed on trainingsmill. Each phase lasted 4 weeks. At week 4 of each phase a standardized exercise test (SET) on treadmill was performed. Blood samples were collected at rest (baseline values), and after 15, 30 and 60 min after SET, according to different phases. Horses were randomly distinguished in two groups of 18 subjects. One group of horses was fed a high fiber diet (HF group) and one group was fed a low fiber diet (LF group). Results showed that in basal conditions diet influenced significantly fT3 values, with the highest concentration in HF, and fT4 values with the highest concentrations in LF group. The time influenced significantly THs in both groups, with the highest values at June for T3, and at October for T4, fT3 and fT4. A significant correlation between T4 and fT4 was observed in basal conditions. In exercise conditions, diet influenced only fT4 values, with thehighest concentrations in LF group. The time influenced significantlyT4 andfT4 concentrations, with the highest values at 15 min for fT4 and at 30 min for T4 values and the lowest values at 60 min for both hormones. Significant correlations between T3 and fT3, as well as T4 and fT4 were observed in exercise conditions. The evaluation of THs output, according to diet contents, could mirror a quantitative and qualitative interplay between thyroid function and increasing exercise level of trained horses.