Iodine intake and excretion in two British towns: aspects of questionnaire validation

One hundred and eighty-nine women aged 25-64 years living in Preston and Southampton completed a postal questionnaire on their usual consumption of the major dietary sources of iodine. A subsample of 56 women collected 24-h urine specimens. Median iodine intakes based on the questionnaire responses and food consumption tables were 99 micrograms and 97 micrograms per day, respectively, but these figures rose to 105 micrograms and 118 micrograms when the women were given the opportunity to revise their original responses and analysed values for milk iodine were used. Median urinary iodine excretion (determined by the dry-ashing method) was 70 micrograms/day in Preston and 76 micrograms/day in Southampton. Although positive associations between intake and excretion were observed, the study lacked sufficient statistical power to validate the questionnaire as a measure of iodine intake. Some problems of questionnaire validation are discussed.