Aggression in normal ranges is necessary to provide a survival of individual, but excessive aggression has negative consequences for both individuals and society. The research of aggression formation mechanisms is very important to understand excessive aggression manifestations. Monoaminergic brain systems have a great influence on the emotions and behavior, including aggression, in humans and animals. The thyroid system influences neurotransmitters (particularly serotonin and norepinephrine). According to scientific literature data, both high impulsive and premeditated aggressions are associated with high neuroticism. Taking into account the above mentioned, the interrelation between blood serum levels of thyroid hormones and catecholamine, neuroticism and physical aggression in men with high and middle neuroticism levels was studied. In total group of participants moderate positive correlations between neuroticism and T4, between neuroticism and T3, negative correlation between T4 and norepinephrine were revealed. In men with high neuroticism level the significant positive correlations between neuroticism and T4, between neuroticism and physical aggression were found. In men with middle neuroticism level only the significant negative correlation between T3 and epinephrine was observed. No significant differences in T3, T4, norepinephrine, epinephrine levels between individuals with high and middle neuroticism levels were found, although difference in T4 level was almost significant. It was higher in men with high neuroticism than in individuals with middle neuroticism. Obtained results indicate the involving thyroid hormones in such personality trait formation as neuroticism. The absence of correlation between thyroid hormones and neuroticism in individuals with middle neuroticism level is possible due to the prevalence or different hormones or mediators in neuroticism development in men with high and middle neuroticism. It is confirmed by our early published results about existence of significant positive correlation between blood serum cortisol content and neuroticism in men with high neuroticism level and strong positive correlation between blood serum estradiol content and neuroticism in men with middle neuroticism level. The existence of negative correlations between T4 and norepinephrine in total group of participants, between T3 and epinephrine in men with middle neuroticism level may be explained by influence of thyroid hormones on beta-adrenergic signaling system and by inverse correlation between the density of adrenoceptors and catecholamine levels. So thyroid hormones participate in the formation of such personality traits as neuroticism, in particular, in the formation of a high level of neuroticism, which is confirmed by the existence of a reliable positive correlation between thyroid hormones and neuroticism in total group and in men with high neuroticism level. The absence of interrelation between thyroid hormone levels and neuroticism in men with middle level of neuroticism together with published data about an existence of correlation between cortisol and neuroticism in persons with high neuroticism and between estradiol and neuroticism in individuals with middle neuroticism indicates the predominance of different hormones in neuroticism development in men with high and middle neuroticism levels.
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