Autoimmune diseases (AD) cover a range of conditions where the immune system mistakenly targets and harms healthy tissues and organs in the body[1, 2]. Symptoms of AD vary based on the specific illness and affected organs, but common indicators encompass fatigue, joint pain, fever, skin issues, digestive problems, and neurological complications[3, 4]. Roughly 5% of the world's population suffer through autoimmune diseases. These conditions remain challenging to treat, burdening patients significantly and holding substantial economic implications. Nevertheless, the complete understanding of the disease's pathogenesis remains elusive[5]. Given the lack of a cure for AD, comprehending its causative factors might aid in decelerating its progression. Some studies[6–8] have found that risk factors for AD are multifaceted, containing genes, environments, and lifestyle. Genetic elements notably contribute, heightening the risk for individuals with a family history of the diseases. Additionally, environment elements, like smoking, disclosure to specific chemicals, infections, and dietary patterns significantly influence the development of those diseases. Lifestyle elements like pressure, exercise, and diet show associations with the onset and progression of AD.
Attention Deficit Hyperactivity Disorder (ADHD) is a chronic neurodevelopmental disorder which representatively emerges during childhood. Its fundamental characteristics involve issues with inattention, excessive activity, and impulsive emotional and behavioral displays[9, 10]. These symptoms persist into adolescence and may continue into adulthood. As per latest epidemiological surveys, the world`s ADHD prevalence among children is estimated at around 7.2%, with roughly half of the patients experiencing clinical symptoms that may persist into adulthood[11].
Many studies[12–14] have claimed the relationships between neurological and psychiatric disorders and abnormalities in the immune system; nevertheless, the relationships keep unsure. Similarly, some immune-associated conditions, like asthma, atopic dermatitis, ulcerative colitis, ankylosing spondylitis, juvenile arthritis, celiac disease, and autoimmune thyroid (AT) diseases have also been linked to ADHD[15–18]. A meta-analysis and systemic review by Chen et al[19]. in 2016, based on 10 studies, revealed an overall increased risk of autism among children whose mothers had autoimmune illnesses. Two population-based researches have been implemented: a discovery was an increased risk of autoimmune diseases among ADHD patients[16], while another study found a correlation between type 1 diabetes and increased risk of phrenoblabias, like ADHD[20]. However, some research[21, 22] did not clearly establish a prominent relationship between AD and ADHD. Therefore, it is imperative to elucidate the causality between ADHD and various types of autoimmune illnesses.
Mendelian randomization (MR) is just one statistical technique applied to epidemiology and genetics, with the objective to determining causalities between risk elements and results[23]. MR is on the grounds of the principles of Mendelian inheritance laws, which describe how genetic variants are assigned at random in the course of meiosis[24]. This approach uses instrumental variables (IVs), particularly single nucleotide polymorphisms (SNPs) that are linked to the risk factor of interest (GERD) genetic variants to detect if the selected risk factors have an obvious causal effect upon the results of interest or not[25]. Without randomized controlled trials (RCTs), MR researches present one alternative tactic for causal intervention[26]. Thus, MR has the superiority over conventional observational researches in that MR lessens the risk of baffling and reverse causality, getting it an advantageous tool for detecting the causality in epidemiological researches[27]. Currently, there hasn't been a Mendelian analysis exploring the causality between ADHD and AD. Therefore, this research aimed at establishing a causality between ADHD and 19 ADs (Autoimmune thyroiditis, Type 1 diabetes, Celiac disease, Vitamin B12 deficiency anemia, Idiopathic thrombocytopenic purpura, multiple sclerosis, Graves' disease, Sjogren's syndrome, Iridocyclitis, Ulcerative colitis, Crohn's disease, Psoriasis, Alopecia areata, Vitiligo, Rheumatoid arthritis (RA), Myasthenia gravis, Systemic sclerosis, Systemic lupus erythematosus, Ankylosing spondylitis) with one two-sample MR approach.