The contribution of total and free iodothyronines to welfare maintenance and management stress coping in Ruminants and Equines: Physiological ranges and reference values

In order to acquire a pattern of thyroid involvement in welfare maintenance in Ruminants and Equines, this review summarizes data concerning the reference values of total and free iodothyronines and their modifications in physiological conditions and in different management conditions (pregnancy, lactation, weaning, growth, isolation, restraint, shearing, confinement and transportation). Thyroidal and extrathyroidal tissues efficiently respond to management practices, giving a differentiated contribution to circulating iodothyronine changes. The hormonal response could be mainly attributed to the intracellular deiodination of T4 to T3. Triiodothyronine (T3) and free iodothyronines (fT3 and fT4) result more responsive to management stress, showing different pattern with species and to various conditions, as to environmental conditions in which activities are performed. Intrinsic seasonal changes of iodothyronines and a significant pregnancy effect for T3 were recorded in mares. Higher, although not significant, T3 and T4 concentrations in barren than pregnant mares were observed in donkeys. A positive significant correlation between T3 and T4 was described only in pregnant donkeys. Moreover, a significant effect of season on T3 and fT3 changes was observed both in pregnant and barren donkeys. A significant lactating effect compared with nonlactating stage for T3 and T4 was recorded in mares. In growing foals, body weight (BW) and age were positively correlated with T3 and negatively correlated with T4, fT4 and fT3. Weaning effects were shown for T3 and fT4 concentrations, indicating that weaning represents a severe stress and the presence of conspecific does not reduce psychological stress in this phase. Lambs showed significant decreased T3 and elevated T4 concentrations two weeks after weaning, with higher concentrations in both males and females compared to 24 h. Significant positive correlations were observed between BW and T4, fT3 and fT4 concentrations in lambs. A T3 decrease was detected after isolation, such as induced by confinement and weaning in lambs. Higher T3 concentration after restraint and shearing than after isolation and significant increases in T4, fT3 and fT4 values after restraint and shearing were recorded. The basal concentrations of fT3 in both the inexperienced and experienced transported horses were significantly higher than in untransported experienced horses. Moreover, increases of T3, T4 and fT4 after short road transportation, and significant correlations between T3 and rectal temperature (RT), body weight (BW) and heart rate (HR), confirmed their important role in coping strategy. Thyroid responsiveness to short transport is similar in domestic donkeys and horses, with a preferential release of T3 in horses. A greatest and constant release of T3 and T4, although differentiated, after simulated transportation and after conventional transport of horses confirmed that the degree of stress induced by confinement and additional stressful stimuli associated to road transportation could differently influence the iodothyronine release. Temperamental Limousin young beef bulls showed lower T4 and fT4 concentrations after prolonged transportation than calm subjects, and a concomitant decrease of circulating ACTH, cortisol, T3 and fT3 concentrations, probably induced by down regulation of HPA axis and cortisol negative feedback. These data reinforce the importance of taking into account the evaluation of iodothyronines, and notably of T3, as markers of welfare and stress and their role in ensuring energy homeostasis and productive and reproductive performances in Ruminants and Equines.