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 T-4 to T-3 and to preferentially increase thyroidal T-3 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 T-4 (FT4), free T-3 (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 were correlated with TSH for the entire group, and the same parameters were segregated by TSH quartile. Results were stratified for body mass index and studied separately in a normal-weight subgroup. Main Outcome Measures: Stepwise correlations of FT4, FT3, and FT3/FT4 ratios with TSH. Results: There was a significant positive linear correlation of TSH with FT3 and FT3/FT4 ratios (R = 0.12; P < .0001 in both), but not with FT4. Conclusion: Within the near-euthyroid range, increasing TSH levels are associated with increasing FT3 levels, without an increase in FT4. This provides in vivo support for TSH enhancing preferentially T-3 production and/or secretion.