Variation of iodine status during pregnancy and its associations with thyroid function in women from Rio de Janeiro, Brazil

Abstract

AbstractObjectiveTo assess iodine status and its effects on maternal thyroid function throughout pregnancy.DesignIn the present prospective cohort study, three urinary samples were requested for urinary iodine concentration (UIC) determinations in both the first and third gestational trimesters. Serum thyrotropin (TSH) and free thyroxine (FT4) were analysed in both trimesters and thyroid antibodies were assessed once.SettingRio de Janeiro, Brazil.ParticipantsFirst-trimester pregnant women (n243), of whom 100 were re-evaluated during the third trimester.ResultsIodine sufficiency was found in the studied population (median UIC=216·7 µg/l). The first- and third-trimester median UIC was 221·0 and 208·0 µg/l, respectively. TSH levels (mean (sd)) were higher in the third trimester (1·08 (0·67)v. 1·67 (0·86) mIU/l;P<0·001), while FT4levels decreased significantly (1·18 (0·16)v. 0·88 (0·12) ng/dl;P<0·001), regardless the presence of iodine deficiency (UIC<150 µg/l) or circulating thyroid antibodies. UIC correlated (β; 95% CI) independently and negatively with age (–0·43; –0·71, –0·17) and positively with multiparity (0·15; 0·02, 0·28) and BMI (0·25; 0·00, 0·50). Furthermore, median UIC per pregnant woman tended to correlate positively with TSH (0·07; –0·01, 0·14). Women with median UIC≥250 µg/l and at least one sample ≥500 µg/l throughout pregnancy had a higher risk of subclinical hypothyroidism (OR=6·6; 95% CI 1·2, 37·4).ConclusionsIn this cohort with adequate iodine status during pregnancy, excessive UIC was associated with an increased risk of subclinical hypothyroidism.