title: TSH enhancement of FT4 to FT3 conversion is age dependent.
abstract: OBJECTIVE
  We previously reported increasing free T3 (FT3) to free T4 (FT4) ratios as thyroid-stimulating hormone (TSH) increases within the normal range in children. It is not known if this phenomenon is age-related among humans, as previously reported in rats. This study examines the relationships between TSH and FT3/FT4 ratios in different ages.
  
  
  METHODS
  Retrospective examination of thyroid tests from patients without thyroid disease from community clinics.
  
  
  METHODS
  Free T3, free T4, and TSH levels from 527 564 sera collected from patients aged 1 year or greater were studied. Exclusion criteria were the following: missing data, TSH greater than 7.5mIU/L, and medications that may interfere with thyroid hormone activity. A total of 27 940 samples remaining after exclusion were stratified by age. Samples with available anthropometric data were additionally stratified for body mass index (BMI). Correlations of TSH to FT4, FT3, and FT3/FT4 ratios by age group were examined.
  
  
  RESULTS
  Up to age 40, for each increasing TSH quartile, FT3 and the FT3/FT4 ratio increased and FT4 decreased significantly (for both FT3, FT4 and FT3/FT4 ratio, P<0.05 for every TSH quartile when compared with the 1st quartile, except FT3 in the 30-40 age group). In older age groups, increasing TSH was not associated with increased FT3/FT4 ratio.
  
  
  CONCLUSIONS
  As TSH levels increase, FT3/FT4 ratios increase until age 40, but this differential increase does not occur in older age groups. This may reflect a decrease in thyroxine (T4) to triiodothyronine (T3) conversion with age, which may be part of the aging process.
authors.affiliation: Clalit Health ServicesJerusalem District, Israel Department of PediatricsShaare Zedek Medical Center, Jerusalem, Israel.
authors.forename: David
authors.initials: D
authors.lastname: Strich
authors.affiliation: Department of Obstetrics and GynecologyHadassah-Hebrew University Medical Center, Jerusalem, Israel.
authors.forename: Gilad
authors.initials: G
authors.lastname: Karavani
authors.affiliation: Clalit Health ServicesJerusalem District, Israel.
authors.forename: Shalom
authors.initials: S
authors.lastname: Edri
authors.affiliation: Department of PediatricsHadassah-Hebrew University Medical Center, Jerusalem, Israel dgillis@hadassah.org.il.
authors.forename: David
authors.initials: D
authors.lastname: Gillis
mesh_terms: D000293:Adolescent; D000328:Adult; D000367:Age Factors; D000368:Aged; D000369:Aged, 80 and over; D002648:Child; D002675:Child, Preschool; D005260:Female; D006801:Humans; D007223:Infant; D008297:Male; D008875:Middle Aged; D012016:Reference Values; D012189:Retrospective Studies; D013960:Thyroid Function Tests; D013972:Thyrotropin / Q000097:blood*; D013974:Thyroxine / Q000097:blood*; D014284:Triiodothyronine / Q000097:blood*; D055815:Young Adult
chemical_list: D014284:Triiodothyronine; D013972:Thyrotropin; D013974:Thyroxine