title: 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 (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.
authors.affiliation: Division of Endocrinology, Department of Medicine.
authors.forename: Annenienke C
authors.initials: AC
authors.lastname: van de Ven
authors.forename: Romana T
authors.initials: RT
authors.lastname: Netea-Maier
authors.forename: H Alec
authors.initials: HA
authors.lastname: Ross
authors.forename: Teun A E
authors.initials: TA
authors.lastname: van Herwaarden
authors.forename: Suzanne
authors.initials: S
authors.lastname: Holewijn
authors.forename: Jacqueline
authors.initials: J
authors.lastname: de Graaf
authors.forename: Bart L A
authors.initials: BL
authors.lastname: Kiemeney
authors.forename: Doorlène
authors.initials: D
authors.lastname: van Tienoven
authors.forename: Jack F M
authors.initials: JF
authors.lastname: Wetzels
authors.forename: Johannes W
authors.initials: JW
authors.lastname: Smit
authors.forename: Fred C G J
authors.initials: FC
authors.lastname: Sweep
authors.forename: Ad R M M
authors.initials: AR
authors.lastname: Hermus
authors.forename: Martin
authors.initials: M
authors.lastname: den Heijer
mesh_terms: D000368:Aged; D000375:Aging; D001323:Autoantibodies / Q000032:analysis; D004032:Diet / Q000009:adverse effects; D005260:Female; D006306:Health Surveys; D006801:Humans; D006980:Hyperthyroidism / Q000097:blood / Q000209:etiology / Q000276:immunology / Q000503:physiopathology*; D007455:Iodine / Q000008:administration & dosage* / Q000172:deficiency / Q000627:therapeutic use / Q000652:urine; D008137:Longitudinal Studies; D008297:Male; D008875:Middle Aged; D009426:Netherlands; D009752:Nutritional Status; D012720:Severity of Illness Index; D012727:Sex Characteristics; D013961:Thyroid Gland / Q000254:growth & development / Q000276:immunology / Q000378:metabolism* / Q000473:pathology; D013972:Thyrotropin / Q000097:blood / Q000378:metabolism; D013974:Thyroxine / Q000097:blood / Q000378:metabolism; D014284:Triiodothyronine / Q000097:blood / Q000378:metabolism
chemical_list: D001323:Autoantibodies; C056626:thyroid microsomal antibodies; D014284:Triiodothyronine; D013972:Thyrotropin; D007455:Iodine; D013974:Thyroxine