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

Eur J Endocrinol. 2013 Nov 22;170(1):49-54. doi: 10.1530/EJE-13-0589. Print 2014 Jan.

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 (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.

MeSH terms

  • Aged
  • Aging*
  • Autoantibodies / analysis
  • Diet* / adverse effects
  • Female
  • Health Surveys
  • Humans
  • Hyperthyroidism / blood
  • Hyperthyroidism / etiology
  • Hyperthyroidism / immunology
  • Hyperthyroidism / physiopathology*
  • Iodine / administration & dosage*
  • Iodine / deficiency
  • Iodine / therapeutic use
  • Iodine / urine
  • Longitudinal Studies
  • Male
  • Middle Aged
  • Netherlands
  • Nutritional Status*
  • Severity of Illness Index
  • Sex Characteristics
  • Thyroid Gland / growth & development
  • Thyroid Gland / immunology
  • Thyroid Gland / metabolism*
  • Thyroid Gland / pathology
  • Thyrotropin / blood
  • Thyrotropin / metabolism
  • Thyroxine / blood
  • Thyroxine / metabolism
  • Triiodothyronine / blood
  • Triiodothyronine / metabolism

Substances

  • Autoantibodies
  • thyroid microsomal antibodies
  • Triiodothyronine
  • Thyrotropin
  • Iodine
  • Thyroxine