Thyrotropin Versus Age Relation as an Indicator of Historical Iodine Intake

Annenienke C van de Ven, Romana T Netea-Maier, Johannes W Smit, Ron Kusters, Jos W J van der Stappen, Claudia J Pronk-Admiraal, Madelon M Buijs, Christian H H Schoenmakers, Stephan G A Koehorst, Monique J M de Groot, Fred C G J Sweep, Ad R M M Hermus, Martin den Heijer

Background In populations with mild iodine deficiency, serum level of thyroid stimulating hormone (TSH) is negatively and serum free T4 (FT4) is positively associated with age. An ongoing decrease of TSH and increase of FT4 can be found after supplementation of iodine. The aim of this study was to investigate whether there are current differences in the relationship 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 elderly is only present in areas with a historically iodine deficiency (RC -0.008, 95% CI -0.009;-0.007). In the historical iodine sufficient population, TSH shows no obvious increase or decrease with age. In both the historically iodine sufficient and deficient populations, FT4 levels were positively associated with age in 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 relationship 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.