title: Thyrotropin versus age relation as an indicator of historical iodine intake.
abstract: BACKGROUND
  In populations with mild iodine deficiency, the serum level of thyrotropin (TSH) is negatively and the serum free thyroxine (FT4) is positively associated with age. An ongoing decrease of TSH and increase of FT4 can be found after iodine supplementation. The aim of this study was to investigate whether there are current differences in the relation between thyroid function and age in relation to differences in iodine intake in the past.
  
  
  METHODS
  Eight medical laboratories in several regions of The Netherlands, which are all iodine sufficient at present but with a difference in iodine status in the past, provided the results of all TSH and FT4 measurements performed from 2006 until 2011, resulting in 330,802 TSH and 103,940 FT4 measurements.
  
  
  RESULTS
  The negative association between TSH and age in the elderly is only present in areas with a historical iodine deficiency (regression coefficients [RC] -0.008, 95% confidence interval [CI] -0.009; -0.007). In the historically iodine-sufficient population, TSH shows no obvious increase or decrease with age. In both the historically iodine-sufficient and iodine-deficient populations, FT4 levels were positively associated with age in the elderly (RC 0.009, 95% CI 0.008; 0.010 and RC 0.008, 95% CI 0.007; 0.010, respectively).
  
  
  CONCLUSIONS
  There are differences in relation between thyroid function and age between populations with differences in iodine intake in the past, despite an adequate iodine status at present. This raises the question whether the present but also historical iodine status of a population should be taken into account when establishing the reference limits of TSH and FT4.
authors: 1Department of Medicine, Division of Endocrinology, Radboud University Medical Centre, Nijmegen, the Netherlands.
authors: Annenienke C
authors: AC
authors: van de Ven
authors: 1Department of Medicine, Division of Endocrinology, Radboud University Medical Centre, Nijmegen, the Netherlands.
authors: Romana T
authors: RT
authors: Netea-Maier
authors: 1Department of Medicine, Division of Endocrinology, Radboud University Medical Centre, Nijmegen, the Netherlands.
authors: Johannes W
authors: JW
authors: Smit
authors: 2Laboratory for Clinical Chemistry and Haematology, Jeroen Bosch Hospital, 's-Hertogenbosch, the Netherlands.
authors: Ron
authors: R
authors: Kusters
authors: 3Department of Laboratory Medicine, Canisius-Wilhelmina Ziekenhuis, Nijmegen, the Netherlands.
authors: Jos W J
authors: JW
authors: van der Stappen
authors: 4Atal-Medial Diagnostic Centers, Hoofddorp, The Netherlands.
authors: Claudia J
authors: CJ
authors: Pronk-Admiraal
authors: 4Atal-Medial Diagnostic Centers, Hoofddorp, The Netherlands.
authors: Madelon M
authors: MM
authors: Buijs
authors: 5Department of Laboratory Medicine, Elkerliek Hospital, Helmond, the Netherlands.
authors: Christian H H
authors: CH
authors: Schoenmakers
authors: 6Department of Laboratory Medicine, Slingeland Hospital, Doetinchem, the Netherlands.
authors: Stephan G A
authors: SG
authors: Koehorst
authors: 7Department of Laboratory Medicine, Amphia Hospital, Breda, the Netherlands.
authors: Monique J M
authors: MJ
authors: de Groot
authors: 8Department of Laboratory Medicine, Radboud University Medical Centre, Nijmegen, the Netherlands.
authors: Fred C G J
authors: FC
authors: Sweep
authors: 1Department of Medicine, Division of Endocrinology, Radboud University Medical Centre, Nijmegen, the Netherlands.
authors: Ad R M M
authors: AR
authors: Hermus
authors: 1Department of Medicine, Division of Endocrinology, Radboud University Medical Centre, Nijmegen, the Netherlands.|9Department of Internal Medicine, Section of Endocrinology, VU Medical Center, Amsterdam, the Netherlands.
authors: Martin
authors: M
authors: den Heijer
mesh_terms: D000293:Adolescent; D000328:Adult; D000367:Age Factors; D000368:Aged; D000369:Aged, 80 and over; D005260:Female; D006801:Humans; D007455:Iodine / Q000172:deficiency*; D016014:Linear Models; D008297:Male; D008875:Middle Aged; D009426:Netherlands; D012189:Retrospective Studies; D013960:Thyroid Function Tests; D013972:Thyrotropin / Q000097:blood*; D013974:Thyroxine / Q000097:blood*; D055815:Young Adult
chemical_list: D013972:Thyrotropin; D007455:Iodine; D013974:Thyroxine