Panic disorder (PD), a prevalent and severe psychological ailment within the anxiety continuum, is distinguished by frequent and unforeseeable episodes of panic, accompanied by a variety of physiological manifestations. It often results in substantial impairment, particularly when accompanied by agoraphobia, and is associated with notable functional morbidity and diminished quality of life. Diagnosis based on International Classification of Diseases 10 (ICD-10) necessitates recurrent panic attacks, accompanied by either apprehension regarding future episodes or the emergence of phobic avoidance. Panic attacks are abrupt and sometimes unforeseen episodes of intense anxiety, often accompanied by physical manifestations such as cardiovascular, otoneurological, gastrointestinal, or autonomic symptoms [1]. A comprehensive meta-analysis revealed that the most common age of onset is during late adolescence or early adulthood [2]. The National Comorbidity Survey Replication (NCS-R) provides prevalence estimates of 2.7% within a 12-month period and 4.7% over the course of a lifetime [3, 4].
Research has shown that risk factors for anxiety disorders include alcohol use, smoking, avoidance behaviors, marijuana use, occupational factors, and negative evaluations of stressful life events. Protective factors that produce some support include physical activity, sports participation, social support, and coping skills. The most researched risk or protective factor was cigarette smoking, which was found to increase the risk of PD development in many prospective studies [5], while much less is known about the role of thyroid function.
At present, the etiology of PD remains unclear, among which the hypothalamic-pituitary-thyroid (HPT) axis is an etiological factor. A large amount of studies have shown that the endocrine system responds to various stressors. Many researchers have explored changes in HPT axis hormones in stress responses [6]. The regulation of thyroid function is a multifaceted process that encompasses not only the thyroid gland, but also the pituitary gland, hypothalamus, and feedback mechanisms. The hypothalamus releases thyrotropin releasing hormone (TRH) acts on the pituitary, rereleasing thyroid stimulating hormone (TSH) acts on the thyroid, which releases free thyroid hormone (FT4). Studies of panic disorder have found evidence of a dulled response of thyroid stimulating hormone (TSH) to thyroid stimulating hormone (TRH) stimulation [7, 8]. Thyroid hormones are essential for neurocognitive development and function, so the link between thyroid dysfunction and panic disorder has been studied over the past few decades [6, 9]. Indeed, both TSH and FT4 have been associated with PD in an observational study [6]. Some researches found initial support for an increased prevalence of thyroid disorders in patients with PD [10, 11]. Because of the high incidence of PD patients, a better understanding of its underlying mechanisms made essential to further improve its prevention and treatment.
Thyroid dysfunction can cause symptoms similar to those seen in people with anxiety disorders, especially panic disorder. Some of the physical symptoms of hyperthyroidism, such as palpitations, shortness of breath, and increased sweating, overlap with what people with panic disorder experience during panic attacks. A link has been suggested between thyroid disease and panic attacks and phobias [12]. However, the extent to which PD patients suffer from thyroid dysfunction remains unclear. Learning more about such dysfunction first means obtaining an estimate of whether a person with PD has (subclinical) thyroid disease. Besides, this means studying subtle changes in individual thyroid function parameters, namely TSH, FT3, and FT4. Doing so may shed light on the complex relationship between PD and thyroid function and help determine whether the aforementioned panic symptoms may also occur as an epiphenomenon of subclinical thyroid dysfunction. Since observational studies are sometimes vulnerable to selection bias, residual confounding, and reverse causality, it is unclear if the reported relationships are causal [13, 14].
Overall, the previous studies point to a connection between thyroid function and panic disorder, but it's important to clarify whether this connection is causative or not. In recent years, with the increasing popularity of genome-wide association studies (GWAS) databases, Mendelian randomization (MR) has attracted a lot of attention. MR can avoid some types of confounding by using genetic variants, which are fixed at conception, to support causal inferences about the effects of changeable risk factors [15]. MR can also yield insights into causality in situations where randomized controlled trials (RCTs) are impractical or insufficient [16], its level of research evidence lies at the junction of RCTs and observational studies [15], as it has the ability to emulate RCTs, and offers a reliable statistical approach utilizing instrumental variables (IVs) to elucidate the causal relationship between exposure and disease [17].
Therefore, in our current investigation, we executed a two-sample MR analysis using the GWAS database to scrutinize the genetic causality interlinking thyroid functionality and panic disorder. The thyroid function we studied included hyperthyroidism, hypothyroidism, FT4, TSH, thyroid peroxidase antibody (TPOAb), and thyroid nodules.