Introduction Thyroid function during the first weeks of life is characterized by hyperactivity as measured by increased radioiodine uptake, increased serum hormonal iodine values, and increased erythrocyte triiodothyronine I131uptake. This neonatal thyroid hyperactivity subsides within one to two weeks and the elevated test results have returned to normal childhood levels by 8 to 12 weeks of age.1There are relatively few reports characterizing and correlating thyroid function tests during childhood subsequent to this period. Radioiodine uptake tests on normal children summarized by Andersen2and Diwany et al3have been, for the most part, late uptake studies (24-48 hours) and have shown considerable variation with geographic area. Adequate comparisons with adult values by similar methodology and characterization of I127excretion have generally not been available. Investigations of serum hormonal iodine values in normal children have been few.4-6Values have been reported to be significantly