Longitudinal trends in thyroid function in relation to iodine intake: ongoing changes of thyroid function despite adequate current iodine status

Objective Several cross-sectional studies on populations with iodine deficiency showed that TSH-levels are negatively associated with age, while in populations with high iodine intake TSH is positively associated with age. The question is whether such an age-thyroid function relation is an ongoing process apparent also in longitudinal studies and whether it reflects an actual iodine deficiency or an iodine insufficiency in the past. Methods In an area with a borderline iodine status in the past, we studied 980 participants of the Nijmegen Biomedical Study. We measured serum TSH, free thyroxine (FT 4 ), total triiodothyronine (T 3 ), peroxidase antibodies, and the urine iodine and creatinine concentration 4 years after our initial survey of thyroid function, in which we reported a negative association between TSH and age. Results Within 4 years, TSH decreased by 5.4% (95% CI 2.5–8.3%) and FT 4 increased by 3.7% (95% CI 2.9–4.6%). Median urinary iodine concentration was 130 μg/l. Estimated 24-h iodine excretion was not associated with TSH, T 3 , change of TSH, or FT 4 over time or with the presence of antibodies against thyroid peroxidase. Only FT 4 appeared to be somewhat higher at lower urine iodine levels: a 1.01% (95% CI 0.17–1.84%) higher FT 4 for each lower iodine quintile. Conclusions In this longitudinal study, we found an ongoing decrease in TSH and increase in FT 4 in a previously iodine insufficient population, despite the adequate iodine status at present. This suggests that low iodine intake at young age leads to thyroid autonomy (and a tendency to hyperthyroidism) that persists despite normal iodine intake later in life.