A 5-year-old child with Down syndrome (DS) diagnosed with mild to moderate obstructive sleep apnea (OSA) and subsequently found to have severe hypothyroidism is presented. Following 3 months of treatment with levothyroxine, she had full resolution of her OSA. This case underscores the importance of routine thyroid function screening in children with DS and demonstrates that OSA in children with DS can be caused by other underlying medical issues that themselves require therapy, such as the hypothyroidism in this child. Although empiric testing of thyroid function is not recommended as part of the routine workup of patients with OSA, when caring for children at higher risk for thyroid dysfunction, such as those with DS, it is important to establish whether or not it is present, as treating it may bring about partial or full resolution of the OSA, as was shown to occur in this patient.