Treatment Trial of Subclinical Hypothyroidism in Down Syndrome

Inclusion Criteria: * Males and females, ages 8 - 20 years * Diagnosis of Down syndrome * Subclinical hypothyroidism: TSH level between 5 - 10 mIU/L, normal T4 * Parental/guardian permission (informed consent) and if appropriate, child assent * Females who are at least 11 years of age or who are menarchal must have a negative urine/serum pregnancy test * Committed to adherence to levothyroxine treatment and study completion Exclusion Criteria: * Pregnancy * Type 1/Type 2 diabetes * Chronic medical conditions or medication use that can affect growth, nutrition, blood glucose, insulin secretion, or thyroid function (such as lithium or certain seizure medications) * Current use of levothyroxine or anti-thyroid hormone * Cyanotic congenital heart disease, or pulmonary hypertension (as described by last echo report in subjects with CHD), or congenital heart disease considered medically unstable by the study cardiologists