TSH concentration within the normal range is associated with cognitive function and ADHD symptoms in healthy preschoolers

Summary Objective Thyroid hormone concentrations outside the normal range affect brain development, but their specific influence on behaviour and mental abilities within normal values is unknown. The objective of this study was to investigate whether thyroid hormone concentrations are related to neurodevelopment and ADHD (attention deficit and hyperactivity disorder) symptoms in healthy preschoolers. Design subjects and measurements Children from two general population birth cohorts in Menorca (n = 289) and Ribera d’Ebre (n = 53), Spain, were assessed in a cross‐sectional study at the age of 4. Thyroid hormones (free T4 and T3) and TSH concentrations were measured and mental and motor development was assessed using McCarthy's scales for neuropsychological outcomes and ADHD‐DSM‐IV for attention deficit and hyperactivity/impulsivity symptoms. Results Children with TSH concentrations in the upper quartile of the normal range performed lower on McCarthy's scales and were at higher risk for attention deficit and hyperactivity/impulsivity symptoms. In the Menorca cohort, a decrease of 5·8 (P