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

Abstract

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 mg/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.

Associations between thyroid function and mortality: the influence of age

“…However, in our population TSH decreases and FT 4 increases with age probably due to mild iodine insufficiency in the past, leading to autonomous function of the thyroid (despite the adequate iodine status at present, achieved after increasing the amount of iodised salt in bakeries in 1982) (30,40). Moreover, in our population, a high-normal TSH level was associated with mortality in the elderly.…”

Prevalence of normal TSH value among patients with autonomously functioning thyroid nodule

“…The latter result challenge the general hypothesis that TS could be not required in patients with normal TSH. Remarkably, most eligible studies were from European countries with current or prior iodine deficiency where autonomous nodular goitres are a frequent finding [13,[28][29][30].…”

“…In contrast, the relationship between thyroid autonomy and TSH levels has not been set, being influenced by the degree of iodine sufficiency and varies widely regionally [9]. In geographic areas that formerly had or still show iodine deficiency, TSH may remain normal even if AFTN is present because, especially early, low synthesis rate of thyroid hormones cannot suppress the TSH secretion [10][11][12][13]. Additionally, in these iodine-deficient areas, euthyroid goitres contain disseminated microscopic areas of 'hot' thyroid tissue carrying constitutively activating TSH receptor mutations, which increase the risk of iodine-induced hyperthyroidism [2,[14][15][16].…”

DIAGNOSIS OF ENDOCRINE DISEASE: High-yield thyroid fine-needle aspiration cytology: an update focused on ancillary techniques improving its accuracy

“…Accordingly, a thyroid scan is usually performed when nodules occur in a background of low-to-normal TSH levels. However, the relationship between thyroid autonomy and TSH levels is dependent on the degree of iodine supply and varies widely from region to region (62). Recent AACE, AME, and ETA guidelines recognize such differences and suggest the use of thyroid scans taking into consideration the iodine supply in different geographical areas.…”