In this study, pregnant women have adequate iodine nutrition based on median UIC. Nonetheless, 38% and 27.8% presented insufficient and more than adequate iodine nutrition, respectively. This finding highlights the evidence of a higher prevalence of iodine deficiency in pregnant women even in iodine-sufficient regions [12, 23].
Considering the WHO epidemiological criteria, the studied population does not present an iodine status that characterizes a public health problem, since less than 50% of the sample (15.8%) presented a UIC < 100 µg/L and less than 20% (3%), a UIC < 50 µg/L [5]. However, there is not a defined criteria for pregnant women, making this interpretation challenging.
In a study conducted by Ferreira et al. [12] in Ribeirão Preto, São Paulo, a non-coastal city of Brazil, 57% of iodine insufficiency was found among 191 pregnant women in the first trimester, while 9.9% had more than adequate iodine nutrition (median UIC: 137.7 µg/L). Mioto et al. [23] detected 52.2% and 4.4% of UIC below 150 µg/L and above 250 µg/L, respectively, in 273 pregnant women from São Paulo (median UIC: 146 µg/L). Saraiva et al. [4] concluded that 48.7% of pregnant women in the first trimester from Rio de Janeiro, a coastal city of Brazil, showed insufficient and 4.5%, excessive UIC (median UIC: 221 µg/L). Machamba (2021) found 22.3% of iodine insufficiency and 8.2% of excessive iodine nutrition in 184 pregnant women from Viçosa (median UIC: 244 µg/L).
It is noted that the prevalence of more than adequate iodine nutrition is low among Brazilian pregnant women, which differs from our finding. Nonetheless, we highlight the methodological and study design differences of the previous studies, such as the method used to evaluate UIC (modified Sandell-Kolthoff reaction versus ICP-MS) and the stage of pregnancy, associated to the fact that Brazil is a heterogeneous country with sociodemographic, geographic and climatic differences, possibly explaining the coexistence of areas with insufficient and adequate median UIC in pregnant women.
The mean of iodine content in household salt samples was 31.4 mg/kg, within the range recommended by ANVISA. It was observed that 71.8% of the samples were in compliance with the legislation, while 12.8% were below and 15.4%, above recommended. One of the sustainability indicators of salt iodization is the proportion of families using properly iodized salt (at least 15 mg/kg) that must be greater than 90% according to the National Program for the Prevention and Control of IDD (Pro-Iodine) [9]. In our study, 87.2% of the samples met this criteria. Nonetheless, due to the small sample size it was not possible to determine the effectiveness of salt iodization policy in the studied city.
In a study conducted by Alves et al. [24], all household salt samples from the schoolchildren in Ribeirão Preto had iodine content within the range recommended by the legislation. In contrast, a previous study conducted in 2010 detected irregular iodine content in salt samples, either less than half or up to three times more than recommended [25].
In accordance with our finding, Saraiva et al. [4] concluded that the most table salt samples contained adequate iodine content, with iodine excess in 18.7% of them. Here, the iodine content in salt could be a predictor of adequate median UIC in pregnant women, although an association has not been detected, corroborating with Azevedo (2019) and de Oliveira Campos et al. [26], probably due to the small sample size.
The main source of iodine in Brazil is the iodized salt, essential to estimate the iodine intake [27]. We concluded that there is an important heterogeneity in the distribution of the iodine content in household salt samples in Brazil, which highlights the need for strengthening the existing salt iodization policy to make a homogenous and adequate iodazed salt for all population groups.
Recently, Milagres et al. [15], researchers of EMDI-Brazil, constructed a table of iodine content in foods using a compilation of international databases from 14 countries. Fish, eggs, sea food, and dairy products are potential sources of iodine worldwide. Nonetheless, the median daily intake of fish, milk and dairy products is low, according to IBGE [28].
On the other hand, ultra-processed foods contributed to 22.7% of the total energy intake in adult population [29, 30]. A study of EMDI-Brazil showed that in natura and minimally processed foods (59.2%) are still the basis of the pregnant women's diet in Ribeirão Preto, São Paulo. However, ultra-processed foods contributed with 28.4% of the total energy intake (Silva, 2021).
Data from Family Budget Survey estimated that the total salt intake is ~ 12 g/day per capita due to the high consumption of processed foods [31]. If we consider this amount of salt, the iodine intake in pregnant women from primary health care units in Ribeirão Preto is ~ 376.8 µg/day (mean iodine content: 31.4 mg/kg). However, a salt intake of 5 g/day, based on the Ministry of Health recommendation, results in an iodine intake of ~ 157 µg/day [32], below the recommendation for pregnant women (250 or 220 µg/day). Actions should be focused on reducing the daily amount of salt and adjusting the content of iodine in salt, considering the possible losses from production to the household level. Strengthening monitoring of the existing policy is critical.
Studies evaluating the source of salt consumption are scarce. This data is relevant because the consumption of seasonings is very common in Brazil, and it may potentially interfere with the amount of iodine intake since seasonings are not directly fortified. In our study, a low consumption of salt in its pure form was observed (20.3%) while there was a high consumption of industrialized seasonings (74.3%). According to Machamba (2021), seasonings were associated to lower UIC. Macedo [33] observed that industrialized seasonings were protective factors while homemade seasonings, risk factors for lower UIC.
Converging with Machamba (2021) and diverging from Macedo [33] we observed that use industrialized seasoning weekly was a predictor of iodine insufficiency regarding to a rare consumption, probably due to the low iodine content compared to salt in its pure form (43 µg/5g of seasoning versus 157 µg/5g of salt). It suggests that iodized salt in manufacturing of processed foods is compromised, since its use in this products is voluntary in Brazil. It was estimated that ~ 25% of processed foods are iodized [26]. Furthermore, WHO recommends in natura foodstuffs and salt in its pure form in the amount up to 5g/day for preparing and cooking food [5].
Personal and environmental factors such as storage, handling practices, and knowledge of iodized salt and IDD may interfere with the iodine stability in salt. Moreover, iodine content in salt may be reduced from its production site to the consumer level [34–36].
The loss of iodine in the salt is more noticeable when packaging is done outside the original container. Salt stored in a covered container was more likely to have adequately iodized salt than in opened containers, since covered containers prevent the salt from being exposed to light and humidity [34, 35]. Here, we found an inverse association that may be explained by other factors not evaluated, such as storage salt area and handling practices which if inadequate the salt may attract moisture and become wet carrying the iodine to the bottom of the container [35]. This may occurs even in covered containers.
We found an association between number of gestations and iodine status. Corroborating our finding, de Zoysa et al. [37] reported that parity was negatively correlated with UIC, but only in the third trimester. Fereja et al. [38] found higher rates of goiter in parous as compared to nulliparous women, suggested that repeated pregnancies could deplete iodine status.
Other variable that must be taken into account is the time interval between the last two pregnancies, which was not evaluated here. Gargari et al. [1] observed that every year added to the time interval between the two most recent pregnancies led to a 20% reduction in low UIC. Possibly, this interval was adequate among secundigravida regarding to multigravida women interfering in UIC in our study.
Regarding iodine supplementation, Murillo-Llorente et al. [39] found that iodized supplements was an important predictor of iodine status in pregnant women. Vongchana et al. [40] found that universal supplementation reduced the prevalence of iodine insufficiency, but it has been associated with excessive iodine nutrition in this group. A randomized controlled trial compared the effectiveness of universal and individualized iodine supplementation in pregnant women. Both strategies reduced the prevalence of iodine insufficiency, however, individualized supplementation prevented more than adequate/excessive UIC, although studies with larger sample sizes are needed [40].
Here, a supplementation with higher content of KI (mean: 200 µg) was associated to more than adequate iodine nutrition. Likewise, Rebagliato et al. [41] found that women who consumed 200 µg/day or more of KI supplement had a higher risk of thyroid dysfunction. These findings deserves attention because iodine excess may lead to an increased risk of subclinical hypothyroidism and isolated hypothyroxinemia in susceptible individuals, besides iodine-induced fetal hypothyroidism (Wolff-Chaikoff effect) [40].
Although KI supplement may be useful to ensure adequate iodine status, care should be taken in countries where salt iodization is mandatory. Moreover, in regions where the UIC is between 150 and 249 µg/L, KI supplementation is not necessary, according to OMS, UNICEF and ICCIDD consensus [42]. Further research is needed to confirm the efficacy and safety of iodine supplementation during pregnancy in areas with adequate iodine intake.
The association between alcohol consumption and iodine status in pregnancy are controversial. A population-based study in Denmark indicated protective effects of moderate alcohol consumption against the development of autoimmune thyroid diseases, which has higher prevalence among women than in men, in part related to pregnancy. On the other hand, alcohol may exert toxic effects on the thyroid gland [43].
Donald et al. [44] found that moderate-to-severe alcohol consumption in pregnancy was associated with alterations in maternal thyroid function, particularly increased serum TSH, decreased serum free T4, and increased serum free T3, a novel and unexpected finding. A possible suggestion is a greater proportion of T4 converted to T3 by increased expression of deiodinase 2 (Dio2) after alcohol exposure. Alcohol is the most important factor that have been documented as having some effects on thyroid size and function, then the control of its consumption is one of the measures of primary prevention of thyroid disorders [45].
As strengths of this study, we highlight the assessment of predictors of insufficient and more than adequate iodine nutrition in pregnant women, and the iodine content in other sources of iodine besides salt. To our knowledge, this is the first study that assessed these factors among pregnant women in the city of Ribeirão Preto. As limitations, we mention the small sample size regarding to household salt samples making it impossible to assess the effectiveness of salt iodization policy in the studied city, and some important data regarding the salt that could interfer on iodine status.