Surprising link found between hyperthyroidism and dark personality traits

New research published in Current Psychology provides evidence linking an overactive thyroid gland to specific personality characteristics known as the “Dark Tetrad.” The study indicates that individuals with hyperthyroidism report higher levels of Machiavellianism, psychopathy, and sadism compared to those with normal thyroid function. These findings suggest that physiological imbalances in the endocrine system may influence socially aversive personality traits.

The thyroid gland plays a central regulatory role within the human body. It releases hormones that control metabolism, energy levels, and heart rate. Beyond these physical functions, the thyroid significantly impacts the brain and nervous system.

Medical professionals have recognized that thyroid dysfunction often accompanies changes in mood and behavior. An overactive thyroid, or hyperthyroidism, frequently presents with symptoms such as restlessness, irritability, and anxiety. In contrast, an underactive thyroid, or hypothyroidism, is often associated with fatigue, mental fog, and emotional flatness.

The authors of the new study noted that the behavioral symptoms of hyperthyroidism overlap with descriptions of antagonistic personality traits. For instance, reduced empathy and impulsive aggression are common in both hyperthyroid patients and individuals with high psychopathy scores. Despite these parallels, there has been little scientific investigation into whether thyroid dysfunction correlates with the Dark Tetrad traits.

The Dark Tetrad refers to a constellation of four socially offensive personality traits. These include Machiavellianism, narcissism, psychopathy, and sadism. While distinct, they share a common core involving emotional coldness and a tendency to prioritize one’s own interests at the expense of others.

“This study was initially motivated by one author’s long-standing diagnosis of Graves’ disease and chronic hyperthyroidism, which provided ongoing exposure to the psychological and interpersonal dimensions of thyroid dysfunction,” explained study authors Or Maimon and Tal Ben Yaacov of Ashkelon Academic College.

“This perspective highlighted a gap in the literature, which has focused primarily on affective symptoms while largely overlooking maladaptive personality traits in the context of thyroid disorders. We sought to address this gap by examining whether chronic thyroid hormonal imbalance is associated with dark personality traits, thereby broadening the psychological framework used to understand thyroid-related conditions.”

To explore this potential connection, the researchers designed a study to compare personality profiles across different thyroid conditions. They aimed to determine if the physiological state of the thyroid could predict variations in these specific personality traits.

The researchers recruited 154 adult participants through online communities. The recruitment process targeted specific health-related support groups to find individuals with diagnosed thyroid issues. The final sample consisted of 140 women and 14 men, with an age range of 18 to 64 years.

Participants were categorized into three distinct groups based on their self-reported medical status. The first group included 49 individuals with hyperthyroidism. The second group consisted of 52 individuals with hypothyroidism. The third group served as a comparison and included 53 individuals who reported no history of thyroid disorders.

Participants with thyroid conditions indicated that they had recently undergone blood tests to verify their diagnosis. This requirement helped ensure that the study captured individuals with active or monitored conditions. The researchers then administered the Short Dark Tetrad (SD4) questionnaire to all participants.

The SD4 is a 28-item assessment tool designed to measure the four dark traits. Participants rated their agreement with statements such as “It is not wise to let people know your secrets” or “Some people deserve to suffer.” Responses were recorded on a five-point scale.

Machiavellianism was measured by items assessing strategic manipulation and cynicism. Narcissism was evaluated through statements reflecting a sense of entitlement and superiority. Psychopathy was assessed via items focusing on impulsivity and callousness. Sadism was measured by the enjoyment of hurting or dominating others.

The results revealed differences in personality scores between the groups. Individuals in the hyperthyroidism group reported higher scores on all four dark traits compared to those in the hypothyroidism group. This finding points to a distinct divergence in personality profiles based on the type of thyroid dysfunction.

When compared to the healthy control group, the hyperthyroidism group continued to show elevated scores. They reported higher levels of Machiavellianism, psychopathy, and sadism. Narcissism scores were higher than the hypothyroidism group but did not statistically differ from the comparison group.

“One notable finding was that associations were observed across multiple dark personality traits rather than being limited to a single dimension,” the researchers told PsyPost. “Drawing on personal experience of coping with chronic hormonal dysregulation, some degree of association was anticipated; however, the breadth of this pattern was unexpected, pointing to a broader involvement of interpersonal and self-related tendencies rather than a trait-specific effect.”

The researchers found no significant differences between the hypothyroidism group and the healthy comparison group. Individuals with underactive thyroids produced personality scores that were statistically similar to those with normal thyroid function. This suggests that the elevation in dark traits is specific to the hyperthyroid state.

The study also examined the relationships between the traits within each group. In the hyperthyroidism group, the four traits were strongly correlated with one another. This indicates a more cohesive or integrated “dark” personality profile among these individuals.

The researchers accounted for demographic variables such as age and sex. Previous research has shown that men generally score higher on dark traits, and these traits tend to decrease with age. The current study confirmed these patterns.

However, even after controlling for age and sex, thyroid status remained a significant predictor of the personality outcomes. This reinforces the idea that the hormonal condition itself contributes uniquely to the expression of these traits.

The authors propose that the physiological mechanisms of hyperthyroidism may drive these psychological outcomes. Elevated thyroid hormones increase metabolic rate and heighten activity in the central nervous system. This state of physiological hyperarousal can lead to emotional instability and reduced impulse control.

Such symptoms align closely with the behavioral manifestations of psychopathy and sadism. The chronic irritability and anxiety associated with hyperthyroidism may exacerbate interpersonal antagonism. Over time, these physiological drivers could shape stable patterns of behavior that are measured as personality traits.

On the other hand, the physiological slowing associated with hypothyroidism might inhibit these traits. The fatigue and emotional blunting common in hypothyroidism do not align with the active hostility or manipulation required for dark traits. This may explain why the hypothyroidism group scored lower than the hyperthyroidism group.

These findings have implications for how clinicians and the public understand behavioral changes in medical patients. The results suggest that hormonal imbalances can affect how individuals think, feel, and interact with others. Recognizing this biological influence may foster greater compassion and understanding in interpersonal relationships.

“The main takeaway is to raise awareness that thyroid hormonal imbalance may affect not only emotional well-being, but also the way people think and process information, as well as how individuals think, feel, and interact with others,” the researchers found. “These patterns are often subtle and may go unrecognized, but they can shape everyday relationships and self-perception. Greater awareness may help individuals and clinicians interpret such changes with more understanding and compassion.”

It is important to note that these effects were observed at the group level. A diagnosis of hyperthyroidism does not imply that a specific individual has a dark personality.

“The effects observed were modest, but statistically significant,” the researchers noted. “Such effects can be practically meaningful when they point to consistent patterns that may accumulate over time or influence everyday functioning. The findings should be interpreted as informative rather than diagnostic.”

“Importantly, the findings should not be interpreted as implying that individuals with thyroid disorders possess problematic or ‘dark’ personalities. The results reflect associations at the group level, not individual character judgments..”

As with all research, there are also limitations to keep in mind. The design was cross-sectional, which means it cannot prove that thyroid dysfunction causes personality changes. It is possible that shared underlying biological factors contribute to both the thyroid condition and the personality traits.

The reliance on self-reported diagnoses is another limitation. Although participants were recruited from patient support groups, the researchers did not verify hormone levels through independent laboratory testing. This introduces a potential for misclassification or inaccuracy regarding the severity of the condition.

The recruitment method may have introduced self-selection bias. Individuals who choose to participate in online studies about health and personality may differ from the general patient population. They might be more health-conscious or more introspective about their psychological state.

Future research aims to address these limitations by using larger, more diverse samples. The authors plan to incorporate objective medical data to confirm diagnoses and hormone levels. Longitudinal studies could also help track how personality traits change over time as thyroid function fluctuates or responds to treatment.

“We are currently extending this work through additional studies that examine thyroid dysfunction in relation to mental health outcomes and personality characteristics,” the researchers said. “To the best of our knowledge, this study represents the first to directly compare individuals with hyperthyroidism, hypothyroidism, and healthy comparison groups, highlighting the importance of examining thyroid-related conditions within a comparative framework rather than focusing on a single clinical group.”

“Developing this concept further, these projects are designed to broaden understanding of how different thyroid-related conditions intersect with psychological functioning in everyday life. Through this ongoing work, we aim to contribute knowledge that can inform research, clinical awareness, and public understanding of this population.”

The study, “Dark personality traits and thyroid dysfunction: a study based on self-reported thyroid hormonal imbalance,” was authored by Or Maimon and Tal Ben Yaacov.