Thyrotropin versus age relation as an indicator of historical iodine intake

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.

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.

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).

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.