Objective: To review the iodine status of women as assessed through National Health and Nutrition Examination Surveys from 1971 to 2002.

Design and Setting: National normative estimates of iodine status of the civilian, non-institutionalized population in the United States of America.

Subjects: Women of reproductive age and pregnant women.

Results: In the United States of America, iodine began to be added to the diet in the 1920s. An excessive iodine intake was documented by the first National Health and Nutrition Examination Survey (NHANES I) in the 1970s which reported a median urinary iodine (UI) concentration of 320 μg l-1. In the NHANES III survey, conducted between 1988 and 1994, the median UI concentration had decreased to 145 μg l-1, while 14.9% of women aged 15-44 years and 6.9% of pregnant women had a UI concentration 50 μg l-1. The concentrations of serum T4 and thyroid-stimulating hormone of women with a low UI concentration did not, however, indicate an iodine deficiency.

Conclusions: Further studies of the association between iodine excretion and biochemical and physiological changes should be undertaken to better understand women's needs for iodine and to develop criteria to monitor them in pregnancy. Because of the potential harm caused by iodine deficiency during pregnancy, we support the use of iodine supplements for all pregnancies while these data are being collected.

Objective: To examine the importance of iodine nutrition during pregnancy.

Design: Review of existing literature of iodine in pregnancy.

Setting: Population surveys and metabolic studies.

Subjects: Pregnant women.

Results: The main changes in thyroid function associated with pregnancy are due to an increase in hormone requirements that begin in the first trimester of gestation. This increase can only be met by a proportional increase in hormone production, something that depends directly upon the availability of iodine. When dietary iodine is lacking, an adequate physiological adaptation is difficult to achieve and is progressively replaced by pathological alterations that occur in parallel with the degree and duration of iodine deprivation.

Conclusions: Iodine prophylaxis should be given systematically to women during pregnancy. In most public health programmes dealing with the correction of iodine deficiency disorders, iodised salt has been used as the preferred means to deliver iodine to households. Iodised salt, however, is not the ideal means of delivering iodine in the specific instances of pregnancy, breast-feeding and complementary feeding because of the need to limit salt intake during these periods. In European countries, presently it is proposed that iodine is given to pregnant women and breast-feeding mothers by systematically administering multivitamin tablets containing iodine in order to reach the recommended dietary allowance of 250 μg iodine day-1.

Objective: To review methods for evaluating iodine deficiency in pregnant women and young infants and to discuss factors to be considered in the interpretation of their results.

Design: Review of the literature regarding the various methods available for assessing iodine status.

Setting: Population surveys and research studies.

Subjects: Pregnant women and young infants.

Results: Several factors to consider when assessing iodine status in pregnant women and young infants include: 1) the urinary iodine (UI) concentration (μg l-1) is not interchangeable with 24 h UI excretion (μg per 24 h); 2) the concentration of iodine in a spot or casual urine sample cannot be used to diagnose iodine deficiency in an individual; 3) a moderate fall in the concentration of serum free T4 during pregnancy is not a sign of maternal iodine deficiency; 4) an increase in the concentration of serum thyroglobulin (Tg) during pregnancy is not a sign of maternal iodine deficiency; 5) a higher concentration of TSH and Tg in cord blood than in maternal blood is not a sign of iodine deficiency in the mother or neonate; and 6) thyroid function in a full-term foetus, a neonate or a small child is not more sensitive to a mild iodine deficiency than in the mother.

Conclusions: If the iodine status of pregnant women and small children is not to be misjudged, the above six factors need to be taken into account.

An inadequate supply of iodine during gestation results in damage to the foetal brain that is irreversible by mid-gestation unless timely interventions can correct the accompanying maternal hypothyroxinemia. Even mild to moderate maternal hypothyroxinemia may result in suboptimal neurodevelopment. This review mainly focuses on iodine and thyroid hormone economy up to mid-gestation, a period during which the mother is the only source for the developing brain of the foetus. The cerebral cortex of the foetus depends on maternal thyroxine (T4) for the production of the 3′,3,5-tri-iodothyronine (T3) for nuclear receptor-binding and biological effectiveness.

Maternal hypothyroxinemia early in pregnancy is potentially damaging for foetal brain development. Direct evidence has been obtained from experiments on animals: even a relatively mild and transient hypothyroxinemia during corticogenesis, which takes place mostly before mid-gestation in humans, affects the migration of radial neurons, which settle permanently in heterotopic locations within the cortex and hippocampus. Behavioural defects have also been detected.

The conceptus imposes important early changes on maternal thyroid hormone economy that practically doubles the amount of T4 secreted something that requires a concordant increase in the availability of iodine, from 150 to 250–300 μg I day− 1. Women who are unable to increase their production of T4 early in pregnancy constitute a population at risk for having children with neurological disabilities. As a mild to moderate iodine deficiency is still the most widespread cause of maternal hypothyroxinemia, the birth of many children with learning disabilities may be prevented by advising women to take iodine supplements as soon as pregnancy starts, or earlier if possible, in order to ensure that their requirements for iodine are met.

Objective: This paper re-evaluates the requirements for iodine during pregnancy, lactation and the neonatal period, and formulates original proposals for the median concentrations of urinary iodine (UI) that indicate optimal iodine nutrition during these three critical periods of life. This paper also discusses the measurements that are used to explore thyroid functions during the same periods.

Design: An extensive and critical review of the literature on thyroid physiopathology during the perinatal period.

Setting: Human studies conducted in various regions throughout the world.

Subjects: Pregnant women, lactating women, and newborns.

Results: The following proposals are made after extensive review of the literature: the requirement for iodine by the mother during pregnancy is 250-300 μg day-1; during lactation the requirement is 225-350 μg day-1; and during the neonatal period the requirement of the infant is 90 μg day-1. The median UI that indicates an optimal iodine nutrition during these three periods should be in the range of 150-230 μg day-1. These figures are higher than recommended to date by the international agencies.

Conclusions: Pregnant women and young infants, but especially the second group, are more sensitive to the effects of an iodine deficiency (ID) than the general population because their serum thyroid-stimulating hormone (TSH) and thyroxine are increased and decreased, respectively, for degrees of ID that do not seem to affect thyroid function in the general population. Systematic neonatal thyroid screening using primary TSH could be the most sensitive indicator to monitor the process of ID control.

Objectives: Monitoring of iodine status during pregnancy, lactation and infancy is difficult as there are no established reference criteria for urinary iodine concentration (UI) for these groups; so it is uncertain whether iodized salt programs meet the needs of these life stages.

Design and Subjects: The method used in this paper was: 1) to estimate the median UI concentration that reflects adequate iodine intake during these life stages; and 2) to use these estimates in a review of the literature to assess whether salt iodisation can control iodine deficiency in pregnant and lactating women, and their infants.

Results: For pregnancy, recommended mean daily iodine intakes of 220-250 μg were estimated to correspond to a median UI concentration of about 150 μg l− 1, and larger surveys from the iodine sufficient countries have reported a median UI in pregnant women ≥ 140 μg l− 1. Iodine supplementation in pregnant women who are mild-to-moderately iodine deficient is beneficial, but there is no clear affect on maternal or newborn thyroid hormone levels. In countries where the iodine intake is sufficient, most mothers have median breast milk iodine concentration (BMIC) greater than the concentration (100-120 μg l− 1) required to meet an infant's needs. The median UI concentration during infancy that indicates optimal iodine nutrition is estimated to be ≥ 100 μg l− 1. In iodine-sufficient countries, the median UI concentration in infants ranges from 90-170 μg l− 1, suggesting adequate iodine intake in infancy.

Conclusions: These findings suggest pregnant and lactating women and their infants in countries with successful sustained iodised salt programs have adequate iodine status.

Objective: To describe studies evaluating urinary iodine excretion during pregnancy and lactation in women living in cities with adequate or more than adequate iodine intake.

Design: Cross-sectional study conducted between 1996 and 1998 in pregnant women and a study of lactating women conducted in 2003.

Settings and Subjects: Pregnant women attending prenatal clinics in four cities in the Islamic Republic of Iran. Urinary iodine excretion and thyroid volume was measured in 403 women. In a second study, 100 lactating women from Taleghani Hospital in Gorgan, Iran were evaluated for thyroid size, and both urinary and breast milk iodine concentrations were determined.

Results: In Rasht city, 84% of pregnant women had a urinary iodine concentration of ≥ 200 μg l-1, while in the other cities this percentage ranged from 45 to 55%. When data were combined for the cities of Ilam, Isfahan and Tehran, where women have an adequate or more than adequate median urinary iodine concentration, 51% of pregnant women had a urinary iodine concentration less than that recommended during pregnancy. In Rasht, where the median urinary iodine concentration indicates an excessive iodine intake, 15.4% of pregnant women had a urinary iodine concentration < 200 μg l-1. The mean urinary iodine concentration in lactating women was 250 μg l-1, and 16% of women had a urinary iodine concentration < 100 μg l-1. Grade 1 goitre was present in 8% of lactating women, and another 8% had grade 2 goitre.

Conclusions: Findings of this study call for further attention to iodine intake during pregnancy and lactation. The currently recommended intake of iodine through universal salt iodisation may not be adequate for pregnant and lactating women, and supplementation during pregnancy and lactation should be further considered in light of the latest recommendations.

Objective: To describe the iodine nutrition of pregnant and lactating women in Hong Kong, where intake is of borderline sufficiency.

Design: Review of cross-sectional and prospective studies.

Setting: China, Hong Kong Special Administrative Region (SAR).

Subjects: Pregnant and lactating women.

Results: Studies of pregnant women in Hong Kong SAR have revealed an increase in the urinary iodine (UI) concentration as pregnancy advances. A significant percentage of women had a sub-normal serum thyroid hormone concentration at full term. Although iodine is concentrated by the mammary gland, 19% of all mothers had low iodine concentrations in their breast milk. The moderate correlation between the concentrations of iodine in breast milk and urine suggests that an adequate maternal urinary iodine concentration cannot reliably indicate that an infant is getting enough iodine in breast milk. Therefore, some breast-fed infants may still be at risk of low iodine intake, and additional iodine supplements, other than salt iodisation, would be warranted in this population.

Conclusions: The currently recommended intake of iodine through universal salt iodisation may not be adequate for pregnant and lactating women, and supplementation during pregnancy and lactation should be further considered in light of the latest recommendations.

Objective: To present data on the relationship between the concentration of thyroid-stimulating hormone (TSH) in whole blood or serum from neonates and the concentration of iodine in their mother's urine collected at birth to contribute to the contention that the recommended iodine intake during pregnancy should be increased.

Design and Setting: Data were provided by current programmes of neonatal screening of congenital hypothyroidism in Bangkok and rural areas of Thailand.

Subjects: A total of 5144 cord serum samples were collected in 2003 and measured for TSH concentrations. Paired samples of blood and urine were collected in 2000 from 203 infants and their mothers and from 1182 infant-mother pairs in 2002-03 in six rural provinces. Iodine was measured in the urine and TSH was measured in cord serum.

Results: The urinary iodine concentration of mothers in rural Thailand is adequate, with a median of 103 μg l-1. However, in 2000, the median urinary iodine concentration of mothers in Bangkok was only 85 μg l-1. The concentration of TSH in whole blood collected on filter paper from neonates was not sensitive enough to be used as a monitoring tool for iodine nutrition in the neonates, as there was no relationship with the concentration of iodine in the urine of the children's mothers. This was in contrast to the concentration of TSH in serum collected from cord blood.

Conclusions: Several conclusions were drawn from this data: 1) Neonatal TSH screening using whole blood collected from a heel prick at 3 days of age is not sensitive enough to assess the iodine nutrition of neonates; 2) Neonatal TSH screening using cord sera can be used to assess iodine nutrition in neonates; 3) The optimum median maternal urinary iodine concentration in Thailand appears to be 103 μg l-1; 4) The criteria proposed by WHO, UNICEF, and ICCIDD to assess iodine nutrition using data on neonatal TSH concentrations should be reassessed; and 5) Neonatal TSH screening can be effectively performed by collecting cord serum in district hospitals in Thailand.