Objective: To test the hypothesis that thyroid functioning would be uniquely associated with sluggish cognitive tempo (SCT) symptoms but not Diagnostic and Statistical Manual of Mental Disorders (4th ed.; DSM-IV) symptoms of ADHD. Method: Serum thyroid stimulating hormone (TSH) was collected from a sample of psychiatrically hospitalized children (N = 570; ages = 6-12), with rates of TSH similar to those found in the normative pediatric population. Caregivers completed narrowband measures of SCT and ADHD symptoms and broadband measures of internalizing/externalizing symptoms. Results: A significant, positive relation was found between TSH concentration and SCT symptoms, but not between TSH and ADHD symptoms. Furthermore, regression analyses indicated that the association between TSH and SCT remained significant after controlling for child demographic characteristics, broadband measures of internalizing and externalizing symptoms, and ADHD symptoms. However, effects were small in magnitude. Conclusion: Although preliminary, results indicate that thyroid functioning may be related to SCT. Given the small effects, however, it is important for future research to examine other endocrine and biological correlates that may inform models of the developmental psychopathology of SCT.