 This study compares two different brands of thyroxine thyroid hormone. Currently, pharmacists may be substituting generic formulations of thyroid hormone without your doctor knowing about this. Although a small difference in thyroid function is not significant in most healthy children, adolescents and adults, in infants and toddlers even a small difference in thyroid function can have important harmful consequences on brain development. The purpose of the present study is to learn whether the difference between brands of thyroid hormone that are currently being substituted is sufficient to cause a difference in thyroid function. 