title: Increases in thyrotropin within the near-normal range are associated with increased triiodothyronine but not increased thyroxine in the pediatric age group.
abstract: BACKGROUND
  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.
  
  
  METHODS
  A large database of pediatric and adolescent thyroid test results drawn in community clinics from children and adolescents without known thyroid disease was analyzed.
  
  
  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.
  
  
  METHODS
  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.
  
  
  CONCLUSIONS
  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 T3 production and/or secretion.
authors.affiliation: Hebrew University Hadassah School of Medicine (G.K.), Jerusalem 91120, Israel; Clalit Health Services (D.S.), Jerusalem 95146, Israel; Shaare Zedek Medical Center (D.S.), Jerusalem 91031, Israel; Health Information Center (S.E.), Clalit Health Services, Jerusalem 95146, Israel; and Hadassah-Hebrew University Medical Center (D.G.), Jerusalem 91120, Israel.
authors.forename: Gilad
authors.initials: G
authors.lastname: Karavani
authors.forename: David
authors.initials: D
authors.lastname: Strich
authors.forename: Shalom
authors.initials: S
authors.lastname: Edri
authors.forename: David
authors.initials: D
authors.lastname: Gillis
mesh_terms: D000293:Adolescent; D000328:Adult; D000367:Age Factors; D001835:Body Weight / Q000502:physiology; D002648:Child; D002675:Child, Preschool; D015331:Cohort Studies; D005260:Female; D006801:Humans; D007223:Infant; D008297:Male; D012016:Reference Values; D013960:Thyroid Function Tests / Q000592:standards* / Q000706:statistics & numerical data; D013972:Thyrotropin / Q000097:blood*; D013974:Thyroxine / Q000097:blood*; D014284:Triiodothyronine / Q000097:blood*; D015854:Up-Regulation; D055815:Young Adult
chemical_list: D014284:Triiodothyronine; D013972:Thyrotropin; D013974:Thyroxine