Increases in Thyrotropin Within the Near-Normal Range Are Associated With Increased Triiodothyronine But Not Increased Thyroxine in the Pediatric Age Group

Context: TSH has been shown in vitro to increase conversion of T4 to T3 and to preferentially increase thyroidal T3 secretion. Whether or not these effects are significant in vivo, other than in obesity, is unclear. Objective: To test whether the incremental relationships between free T4 (FT4), free T3 (FT3), and TSH are compatible with TSH enhancement of a preferential increase in serum FT3. Design and Setting: A large database of pediatric and adolescent thyroid test results drawn in community clinics from children and adolescents without known thyroid disease was analyzed. Patients and Methods: Results of FT3, FT4, and TSH were studied anonymously. They were crossed with electronic charts to exclude a history of positive thyroid autoantibodies and use of thyroid hormone preparations, antithyroid medication, or drugs known to affect thyroid function. All samples from patients appearing more than once in the database were removed. After exclusions, 3276 samples remained. FT4, FT3, and the FT3/FT4 ratios ...