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 (FT4), 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 FT4 increased by 3.7% (95% CI 2.9-4.6%). Median urinary iodine concentration was 130 mu g/l. Estimated 24-h iodine excretion was not associated with TSH, T-3, change of TSH, or FT4 over time or with the presence of antibodies against thyroid peroxidase. Only FT4 appeared to be somewhat higher at lower urine iodine levels: a 1.01% (95% CI 0.17-1.84%) higher FT4 for each lower iodine quintile. Conclusions: In this longitudinal study, we found an ongoing decrease in TSH and increase in FT4 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.