<p>Thyroid hormonal imbalance can manifest as hypothyroidism, involving reduced hormone secretion, or hyperthyroidism, involving elevated secretion. Both conditions have been associated with disruptions in emotional regulation, cognitive functioning, and behavioral control. To the best of our knowledge, no prior study has examined the association between thyroid dysfunction and dark personality traits. Therefore, in this study, Machiavellianism, narcissism, psychopathy, and sadism were measured using a self-report questionnaire, and participants also provided information regarding their thyroid hormonal status. The aim was to explore potential differences between groups in their self-reported levels of these dark personality traits. A total of 154 adults (140 women, 14 men; age range = 18–64) completed an online survey comprising a sociodemographic questionnaire and the Short Dark Tetrad (SD4). Based on participants’ self-reports, 49 were categorized as having hyperthyroidism, 52 as having hypothyroidism, and 53 as not having any thyroid disorder. Significant between-group differences emerged across all four traits. Individuals in the hyperthyroidism group reported higher scores on Machiavellianism, narcissism, psychopathy, and sadism relative to those in the hypothyroidism group, and higher scores on Machiavellianism, psychopathy, and sadism compared to the comparison group. These preliminary findings suggest that elevated thyroid hormone levels may be associated with a personality profile marked by greater antagonism and reduced empathic functioning. The results contribute to the limited body of knowledge on the psychological correlations of thyroid dysfunction and underscore the importance of further investigation into its potential role in shaping maladaptive personality traits.</p>