Depression is a prevalent and substantial medical issue on a global scale, affecting approximately 6 % of the global population (Salk et al., 2017). Simultaneously, depression constitutes a significant contributor to disability and socioeconomic strain on a global scale (Vos et al., 2012). Depression is currently recognized as a significant public health concern, presenting considerable challenges. The global incidence of diagnosed depression has been consistently rising, highlighting the urgency to address this issue (Crockett et al., 2020).

Thyroid dysfunction is a highly prevalent endocrine disorder worldwide, with a global occurrence rate of hypothyroidism estimated to be approximately 4 % - 5 %(Koyyada and Orsu, 2020). Hypothyroidism is recognized as an important cause or risk factor for depression. There exists a certain degree of symptom overlap between hypothyroidism and depression. However, recent research has delved into a more specific connection between these two disorders. Nevertheless, the exact causal relationship between them has not been definitively established. A meta-analysis comprising 25 studies revealed a significant association between hypothyroidism and clinical depression (Bode et al., 2021). In contrast, a separate meta-analysis yielded a limited and statistically insignificant correlation between hypothyroidism and depression (Wildisen et al., 2020). However, it is important to note that this analysis was based on a small sample size of only six studies. Consequently, additional research is necessary to fully investigate the relationship between these two conditions.

Recent research has indicated that testosterone may exert a significant influence on the association between hypothyroidism and depression. Testosterone is a pivotal factor in the pathophysiology of diseases, the determination of health outcomes, and the maintenance of physiological homeostasis. Furthermore, apart from its role in the regulation of male sexual development, sperm production, and sexual function (Bhasin et al., 2018), testosterone has been discovered to exert a wider array of physiological effects. These effects encompass the regulation of various systems such as cardiovascular, metabolic, hepatic, immune, and notably, the psychiatric system (Kloner et al., 2016; Morford et al., 2018; Morssinkhof et al., 2020). Interestingly, the symptoms of testosterone deficiency and thyroid dysfunction often overlap. Studies have shown that primary hypothyroidism not only leads to lower sex hormone binding globulin (SHBG) and total testosterone but also to lower free testosterone in about 60 % of hypothyroid men (Meikle, 2004). The impact of testosterone on the development of depression varies between males and females, necessitating additional scholarly discourse. Earlier studies demonstrated a significant association between lower circulating levels of total testosterone and increased severity of major depression in men (Yesavage et al., 1985). Ongoing observational, cross-sectional, and longitudinal investigations persist in examining this inquiry, wherein certain studies have indicated a negative correlation between levels of circulating testosterone and depressive symptoms in males, while others have not observed such an association (Amini et al., 2023; Li et al., 2022; Määttänen et al., 2021). Previous studies on women have also generated conflicting findings regarding the relationship between depression and serum testosterone levels. Some research indicates that women with depression have lower levels of testosterone in their bloodstream compared to those without the condition (Kumsar et al., 2014), while other studies have reported higher serum testosterone levels in women with depression (Aswathi et al., 2015). Further research is necessary to address these contradictions and controversies.

Traditional observational epidemiologic studies can only speculate about correlations between events. In contrast to randomized clinical trials, observational epidemiological studies cannot infer causality due to the interference of potential confounders. Mendelian randomization (MR) is a relatively reliable alternative method for inferring causal associations (Byrne et al., 2017). The approach considers genetic variants as instrumental variables (IVs) for a particular trait. By assuming that genetic variants are randomly assigned prior to birth, this method enables the investigation of causal associations without the inherent biases associated with conventional observational studies (Gusev et al., 2016).

Here, we used univariate and multivariate MR analyses in two samples to assess the causal relationships between hypothyroidism, testosterone traits, and three depression traits. IVs on hypothyroidism were obtained from publicly available summary statistics from the most recent large-scale global census, which included 213,990 Finnish individuals. Genetic instruments stratified by sex for serum total testosterone, SHBG and bioavailable testosterone were created using Genome-Wide Association Study (GWAS) data from the UK Biobank Study, which all contain probable sample sizes of >174,519 cases. For depression, we used three different traits of depression (Howard et al., 2018): Self-reported past help-seeking for problems with ‘nerves, anxiety, tension or depression’ (referred to as ‘broad depression’), self-reported depressive symptoms with associated impairment (referred to as ‘probable MDD’), and ‘ICD-coded MDD’ identified from the International Classification of Diseases (ICD)-9 or ICD-10 hospital code. The broad depression is expected to encompass various personality and psychiatric disorders, while the probable MDD and ICD-coded MDD present more comprehensive definitions for depression. The respective datasets consist of 322,580, 174,519, and 217,584 individuals from Europe. In addition to univariate MR analyses, multivariate MR analyses were conducted to assess the direct causal effects of hypothyroidism and total testosterone levels on depression. Sorting out the complex causal relationship between hypothyroidism, testosterone, and depression will be important for the development of targeted public health policies as well as the development of diagnostic and treatment strategies and protocols for existing testosterone therapies.