Obesity and thyroid function are closely related. Thyroid hormones are involved in the regulation of metabolism, thermogenesis, food intake, and fat oxidation. In obese children the most frequent hormonal abnormalities are slight hyperthyrotropinaemia and moderate increases in total T3 and/or fT3 concentrations. Those abnormalities are usually considered a cause of obesity, but according to recent studies, they should actually be considered an adaptation process aimed at increasing resting energy expenditure and total energy expenditure. Those abnormalities do not require any treatment and normalise after substantial weight loss. The mechanisms of those changes are dependent on leptin, thyroid hormone resistance, and mitochondrial dysfunction. The present paper describes the above-mentioned mechanisms based on the latest research. We also present a review of some recent original studies evaluating thyroid function in overweight and obese children, including thyroid ultrasound. A thyroid ultrasound scan in obese children frequently shows increased thyroid volume, which correlates with moderately increased TSH levels and a hypoechoic pattern typical of autoimmune thyroiditis, but without antithyroid autoantibodies. Alterations of thyroid function in overweight and obese patients cause an increase in energy expenditure, which facilitates weight loss and prevents further weight gain. Therefore, normalisation of TSH and fT3 after weight loss could explain difficulties in maintaining reduced weight.