Evaluation of a semi-quantitative food frequency questionnaire to estimate iodine intake

OBJECTIVE: To evaluate a food frequency questionnaire (FFQ) used to assess the dietary intake of iodine. DESIGN: The iodine intake determined by the FFQ was compared with 4-day dietary records and with iodine excretion in 24 h urine samples in a subgroup of participants in a cross-sectional study of iodine intake and thyroid diseases in Denmark. Furthermore, the intake of fish determined from the FFQ was compared with the intake of fish from a simple record kept for 3 months. SUBJECTS: Women aged 25-30 y and 60-65 y. RESULTS: Median iodine intake was similar when determined from the FFQ and from dietary records and the correlation between these measures was 0.52 (P < 0.001). Iodine intake was higher than iodine excretion (P < 0.001). The cross-check questions in the FFQ (for example the question 'How often did you get any kind of fish?') underestimated the intake. In contrast, the intake of a specific fish tended to be overestimated by the FFQ when compared with the 3 month record of fish intake. CONCLUSION: The FFQ can be used to classify subjects into low and high iodine intake groups, but the level of iodine tends to be overestimated.