Attention deficit hyperactivity disorder (ADHD) is one of the most common neurodevelopmental disorders, affecting approximately 5–7% of children worldwide and often continuing into adulthood. In recent years, rates of ADHD diagnosis have increased, likely due to greater awareness and improved screening methods. ADHD is characterized by symptoms of inattention, hyperactivity, and impulsivity, which can negatively impact academic performance, social relationships, and overall quality of life. Early identification and treatment have been shown to improve long-term symptom severity and functional outcomes, emphasizing the importance of detecting ADHD as early as possible. Recent research suggests that ADHD is not solely a behavioral disorder but also has measurable biological components. Immune signaling molecules called cytokines play an important role in brain development and neural function and have been linked to several neurodevelopmental and psychiatric disorders. Some inflammatory biomarkers have been associated with ADHD symptoms, suggesting that biological markers could help support diagnosis. Because current ADHD diagnosis relies heavily on subjective behavioral assessments, identifying reliable biological markers could improve diagnostic accuracy and allow for earlier detection and intervention. The present study aimed to investigate the relationship between inflammatory biomarkers and ADHD symptoms by measuring levels of insulin-like growth factor 1 (IGF-1), a cytokine involved in immune signaling and brain development. Human nail samples were collected and analyzed for IGF-1 levels and paired with ADHD symptom severity measured using the Adult ADHD Self Report Scale (ASRS-6). This study examined whether IGF-1 levels were associated with overall ADHD severity and whether IGF-1 was more strongly related to inattentive or hyperactive aspects of the disorder.