Thyroid function and prevalence of anti-thyroperoxidase antibodies in a population with borderline sufficient iodine intake: Influences of age and sex

Background: We present a large European population-based study of thyroid function, performed in a population with longstanding borderline sufficient iodine intake. Methods: The Nijmegen Biomedical Study is a population-based survey conducted in the eastern part of The Netherlands. Randomly selected inhabitants received a postal questionnaire on lifestyle and medical history, which was filled out by 9371 individuals (41.7%). We measured serum thyroid-stimulating hormone (TSH), free thyroxine (FT₄), and anti-thyroperoxidase antibodies (TPOAbs) in 6434 responders. A. reference population of 5167 individuals was selected by excluding those at risk for thyroid disease. Results: Overt thyrotoxicosis was found in 0.4% of the total population and subclinical thyrotoxicosis in 0.8%. Overt hypothyroidism was found in 0.4% and subclinical hypothyroidism in 4.0%. In individuals older than 60 years, mean FT₄ concentrations increased with age. Mean TSH decreased with age, from 1.46 mIU/L at 18-24 years to 1.07 mIU/L after 85 years. The mean TSH in the total population did not differ from the mean TSH in the reference population; the exclusion of those at risk for thyroid disease, however, lowered the upper limit of the TSH reference interval considerably. In the total population, 8.6% of males and 18.5% of females had positive TPOAbs. The presence of TPOAbs was associated with abnormally high and low TSH concentrations. Conclusion: In inhabitants of the eastern part of The Netherlands, serum TSH gradually decreases with age, whereas after age 60, serum FT₄ increases, possibly because of the development of thyroid autonomy after longstanding borderline sufficient iodine intake.