Introduction
Pregnancy is accompanied by profound alterations in economy of the thyroid gland which may lead to a relative iodine deficiencyReference Bauch, Meng, Ulrich, Grosse, Kempe, Schonemann, Sterzel, Seitz, Mockel and Weber1. In iodine-deficient populations, pregnancy therefore acts to disclose an underlying lack of iodine. This leads to goitre in the mother and neonate, and to subsequent physical and intellectual impairment of the child, but it can easily be prevented by iodine supplementationReference Glinoer, De Nayer, Delange, Lemone, Toppet, Spehl, Grun, Kinthaert and Lejeune2–Reference Glinoer4. Studies have shown that many women are iodine deficient during pregnancy not only in iodine-deficient regions, but also in iodine-sufficient areasReference Hollowell, Staehling, Hannon, Flanders, Gunter, Maberly, Braverman, Pino, Miller, Garbe, DeLozier and Jackson5.
In the Islamic Republic of Iran, an effective national programme of dietary iodine supplementation by means of universal salt iodisation has resulted in the sustained prevention of iodine deficiency disorders (IDD)Reference Azizi, Sheikholeslam, Hedayati, Mirmiran, Malekafzali and Kimiagar6 so that the Eastern Mediterranean Office of the World Health Organization has classified Iran as an IDD-free country7. This paper describes studies of urinary iodine concentrations during pregnancy and lactation in women living in cities who have an adequate or a more than adequate iodine intake.
Pregnant women and schoolchildren
A cross-sectional study was performed in the cities of Ilam, Isfahan, Rasht and Tehran, between 1996 and 1998. The study populations were pregnant women and schoolchildren. A total of 403 healthy pregnant women who attended prenatal clinics, and 438 schoolchildren, aged 8–10 years, randomly selected from primary schools of each city, were recruited to take part in this study. A sample of urine was collected in the morning from each subject to estimate the urinary iodine concentration using a digestion method. The criteria given by the WHO and the ICCIDD were used to classify the iodine content of each sample8. The volume of the thyroid was measured ultrasonically according to the method of Brunn et al. Reference Brunn, Blocjk, Ruf, Bos, Kunze and Scriba9 using a scanner (Dornier, Germany) with a 7.5 MHz transducer.
Table 1 shows the median urinary iodine concentrations of pregnant women and schoolchildren in Ilam, Isfahan, Rasht and Tehran. In Rasht, 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%. This means that in the cities of Ilam, Isfahan and Tehran, 55, 45 and 54% of women, respectively, had a urinary iodine concentration of ≤ 199 μg l− 1. When data were combined for the three cities of Isfahan, Ilam and Tehran, 4, 5, 18 and 24% of pregnant women had a urinary iodine concentration of < 50, 50–99, 100–149 and 150–199 μg l− 1, respectively.
Table 1 Urinary iodine concentration (μg l−1) in pregnant women and schoolchildren of four cities in the Islamic Republic of Iran.
Table 2 shows the mean and median urinary iodine concentrations of women in each of the three trimesters of pregnancy, and in non-pregnant, control women. Table 2 also shows the mean thyroid volume of each of these groups of women. The urinary iodine concentration of women in the third trimester was significantly lower than in control women (P < 0.02). However, there was no significant difference in urinary iodine concentration in pregnant women among the three trimesters of pregnancy.
Table 2 Urinary iodine concentration and thyroid volume in each trimester of pregnancy and in control, non-pregnant women in Isfahan.
Table 2 also shows that, of the non-pregnant women, only 10% had a urinary iodine concentration less than the minimum adequate level of 100 μg l− 1, while 45% of pregnant women had values less than minimum recommended concentration during pregnancy of 200 μg l− 1.
Lactation
From April to June 2003, lactating mothers referred to the Taleghani Hospital in Gorgan were randomly selected. The size of the thyroid was determined by palpation and was classified into grade 1 or 2 according to the WHO criteria8. Samples of urine and breast milk were collected from 100 women. After acid digestion of urine samples and alkaline incineration of milk samples, the concentration of iodine was determined using the Sandell–Kolthoff colorimetric methodReference Sandell and Kolthoff10.
The standard error of the mean (SEM) of the urinary iodine concentration was 250 (12.3) μg l− 1 with 96% CI of 226–274, the median was 259 μg l− 1, and the range in values was 35–519 μg l− 1. Table 3 shows that 16% of women had a urinary iodine concentration of < 100 μg l− 1, 13% were in the range 50–99 μg l− 1 and 3% were in the range 20–49 μg l− 1. A goitre of grade 1 was present in 8% of women, and another 8% had a grade 2 goitre.
Table 3 Distribution of the concentration of iodine in urine according to the breast milk iodine concentration of 100 lactating mothers in Gorgan, Iran.
The SEM of the breast milk iodine concentration was 117 (10.3) μg l− 1 with 96% CI of 97–137, the median was 93 μg l− 1, and the range in values was 17–696 μg l− 1. The milk iodine concentration was < 50 μg l− 1 in 19% of women, it ranged from 35 to 49 μg l− 1 in 13% of women; from 20 to 34 μg l− 1 in 3% of women and was < 20 μg l− 1 in another 3% of women. The study found that 11% of women had a milk iodine concentration of 20–29 μg l− 1 and 3% had values ≥ 30 μg l− 1. Of the women who had a milk iodine concentration of < 50 μg l− 1, 11% had a grade 1 goitre and 8% had a grade 2 goitre.
The Fig. 1 shows that there was a statistically significant correlation between the concentration of iodine in the breast milk and iodine in the urine of mothers (r = 0.44, P < 0.001).
Fig. 1 Scatter plot of concentration of iodine (μg l− 1) in paired urine and breast milk samples from 100 lactating women in Gorgan, Iran in 2003 and linear regression line (r = 0.44, P < 0.001).
Discussion
The data presented here clearly show that in Ilam, Isfahan and Tehran, where women have an adequate or more than adequate median urinary iodine concentration ranging from 186 to 212 μg l− 1, 51% of pregnant women have a urinary iodine concentration less than that recommended during pregnancy, and 9% have a urinary iodine concentration < 100 μg l− 1, which might be considered to indicate a severe iodine deficiency during pregnancy. In Rasht, where the median urinary iodine was 338 μg l− 1, indicating an excessive iodine intake according to the WHO criteria, 15.4% of pregnant women had a urinary iodine concentration < 200 μg l− 1.
The results also indicate that, although the median urinary iodine concentration of lactating mothers indicated a sufficient consumption of iodine, the concentration of iodine in breast milk was still inadequate in some women, so their infants could be at risk of iodine deficiency. The median and 95% confidence intervals of the urinary iodine concentration in lactating women were within normal range, and a large proportion showed a more than adequate or excessive concentration of urinary iodine.
Although iodine is avidly concentrated by the mammary gland, unexpectedly, 19% of all mothers had low iodine concentration in their breast milk. In addition, both the median and the mean concentrations of iodine in breast milk were lower than the newly recommended adequate iodine intake for infants i.e. 110–130 and 140–180 μg l− 1, respectively.
The moderate correlation between the concentrations of iodine in breast milk and urine illustrates this discrepancy and suggest that an adequate maternal urinary iodine concentration cannot reliably indicate that an infant is getting enough iodine in breast milk. Therefore, it may be concluded that some breast-fed infants are still at risk of a low iodine intake and additional iodine supplements, other than salt iodisation, would be warranted in this population. Increasing the amount of iodine in salt seems not to be a reasonable method to achieve this because of the large number of urine samples that have adequate or excessive concentrations of iodine and previous reports that a high intake can increase the risk of side effects. Supplementation of individual infants using potassium iodine or iodine-including multivitamin preparations may be consideredReference Bazafshan, Mohammadian, Ordookhani, Abedini, Davouday, Pearce, Hedayati, Azizi and Braverman11.
In conclusion, the 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 the lights of the latest recommendationsReference Azizi, Aminorroya, Hedayati, Rezvanian, Amini and Mirmiran12.
