RESUMO
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 (T3), 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 2.58.3%) and FT4 increased by 3.7% (95% ci 2.94.6%). median urinary iodine concentration was 130 µg/l. estimated 24-h iodine excretion was not associated with TSH, T3, 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.
