Background In adults, obesity is associated with abnormalities of thyroid function; there are fewer studies in pediatric cohorts. Objectives To examine associations of weight and adiposity with indices of thyroid function and thyroid-related metabolic factors in children. Design/Methods A sample of 1203 children without obesity (BMI<95th percentile; n=631) and with obesity (BMI≥95th percentile; n=572), age 5-18y, had height and weight measured (to calculate BMI-Z score for age and sex) and had blood collected in the morning for thyroid-stimulating hormone (TSH), free thyroxine (FT4), and leptin. A subset (n=829) also underwent measurement of fat mass by dual-energy x-ray absorptiometry. Analyses examined associations of TSH and FT4 with adiposity and obesity-related conditions, accounting for sociodemographic factors. Results TSH was positively related to BMIz and fat mass (both p-values <.001). FT4 was negatively related to BMIz and fat mass (both p-values <.001). TSH was positively correlated to leptin (p=.001) even after accounting for fat mass. Conclusions Pediatric obesity is associated with higher TSH and lower FT4 concentrations and with a greater prevalence of abnormally high TSH. Leptin concentrations may in part explain obesity’s effects on thyroid status, perhaps through leptin’s effects on TSH secretion.