A survey of iodine intake and thyroid volume in Dutch schoolchildren: reference values in an iodine-sufficient area and the effect of puberty

Background: Iodine deficiency and endemic goiter have been reported in the past in The Netherlands, especially in the southeast. Objective: To evaluate iodine intake and thyroid size in Dutch schoolchildr en, contrasting those living in a formerly iodine-deficient region in the e ast (Doetinchem) with those living in an iodine-sufficient region in the we st (Amsterdam area). Design: Cross-sectional survey of 937 Dutch schoolchildren aged 6-18 years, of whom 390 lived in the eastern and 547 in the western part of the countr y. Methods: Thyroid size was assessed by inspection and palpation as well as b y ultrasound. Iodine intake was evaluated by questionnaires on dietary habi ts and by measurement of urinary iodine concentration. Results: Eastern and western regions were similar with respect to median ur inary iodine concentration (15.7 and 15.3 mug/dl. NS, Mann-Whitney U test), goiter prevalence by inspection and palpation (0.8 and 2.6%, P = 0.08, chi -squared test), and thyroid volumes. The P97.5 values of thyroid volumes pe r age and body surface area group were all lower than the corresponding sex -specific normative WHO reference values. Iodized salt was not used by 45.7% of households. Daily bread consumption w as five slices by boys and four slices by girls. Weekly milk consumption wa s 3 liters by boys and 2 liters by girls, Seafish was consumed once monthly . From these figures we calculated a mean daily iodine intake of 171 mug in boys and 143 mug in girls, in good agreement with the measured median urin ary concentration of 16.7 mug/dl in boys and 14.5 mug/dl in girls. The sex difference in iodine excretion is fully accounted For by an extra daily con sumption of one slice of bread (20 mug I) and one-seventh of a liter of mil k (8.3 mug I) by boys. Thyroid volume increases with age, but a steep increase by 41% was observed in girls between 11 and 12 years, and by 55% in boys between 13 and 14 yea rs. coinciding with peak height velocity, Girls have a larger thyroid volum e at the ages of 12 and 13 years, but thyroid volume is larger in boys as o f the age of 14 years. Conclusions: (1) Iodine deficiency disorders no longer exist in The Netherl ands, (2) Bread consumption remains the main source of dietary iodine in Th e Netherlands; the contribution of iodized table salt and seafish is limite d, (3) The earlier onset of puberty in girls renders their thyroid volume l arger than in boys at the age of 12-13 years, but boys have a Larger thyroi d volume as of the age of 14 years.