We conclude that in a sample of Danish children and adolescents with obesity, we found a prevalence of damaging or unresolved MC4R mutations of 2.5% and showed that at baseline carriers of MC4R mutations had higher fasting plasma concentrations of TSH, and lower fasting plasma concentrations of T4 than noncarriers. The gene discussed is MC4R; the disease is obesity due to melanocortin 4 receptor deficiency.