Longitudinal trends in thyroid function in relation to iodine intake: ongoing changes of thyroid function despite adequate current iodine status Annenienke C van de Ven 1 , Romana T Netea-Maier 1 , H Alec Ross 2 , Teun A E van Herwaarden 2 , Suzanne Holewijn 3 , Jacqueline de Graaf 3 , Bart L A Kiemeney 4,5 , Doorle ` ne van Tienoven 2 , Jack F M Wetzels 6 , Johannes W Smit 1 , Fred C G J Sweep 2 , Ad R M M Hermus 1 and Martin den Heijer 1,4,7 1 Division of Endocrinology, Department of Medicine, 2 Department of Laboratory Medicine, 3 Division ofVascular Medicine, Department of Medicine, 4 Department for Health Evidence, 5 Department of Urology, 6 Department of Nephrology, Radboud University Nijmegen Medical Centre, PO Box 9101, 6500 HB Nijmegen, The Netherlands and 7 Section of Endocrinology, Department of Internal Medicine, VU University Medical Center, Amsterdam, The Netherlands Correspondence should be addressed to A C van de Ven Email A.vandeVen@endo.umcn.nl 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. European Journal of Endocrinology (2014) 170, 49–54 Introduction Iodine is an essential micronutrient and an important component of thyroid hormones. Iodine deficiency can cause thyroid dysfunction, goiter, and cretinism (1). Monitoring the iodine status and maintaining an optimal iodine intake are very important to prevent brain damage in newborns and thyroid function disorders at all ages. Iodine deficiency remains a global public health problem (2). To assess the iodine status of a population, the median European Journal of Endocrinology Clinical Study A C van de Ven and others Thyroid function and iodine status 170 :1 49–54 www.eje-online.org Ñ 2014 European Society of Endocrinology DOI: 10.1530/EJE-13-0589 Printed in Great Britain Published by Bioscientifica Ltd.