Thyroid hormone pattern and aggressiveness. (Isolated mice compared with dominant and submissive)

The aggressiveness induced by the forced isolation of animals may be mediated by endocrine changes. The present study focuses on the possible mediation of the pituitary-thyroid axis in the mouse (Mus musculus). The blood levels of the hormone fractions examined (T4, T3 and rT3) in isolated animals showed a significant increase in comparison with the individuals which were classified from an ethological point of view as submissive, whereas in comparison with the individuals which were classified as dominant, no difference was found for T4 and rT3, but a significant increase of T3 occurred. The choice of the categories of comparison (dominant and submissive as opposed to isolated mice) derives from the hypothesis, which has already been confirmed experimentally on an etiological level, that in this species the isolated individuals are homologous to socially dominant animals as far as aggressiveness is concerned. The endocrinological results obtained in this study support this hypothesis. Thus it is concluded that, in the species under study, isolation induces pituitary-thyroid axis activation which is similar to that found in dominant individuals and at least partially responsible for the isolation aggressiveness. The mediation of the action of TRH on the central nervous system in this phenomenon is suggested as an interesting hypothesis.