Factors associated with inadequate urinary iodine concentration among pregnant women in Mbeya region Tanzania.

Introduction

Iodine deficiency is a significant global public health concern.¹ This element is found in hormones produced by the thyroid gland namely, triiodothyronine (T3) and thyroxine (T4),^1,2 and must be consumed in the diet as it cannot be made naturally in the body.¹ A diet low in iodine results in deficiencies that can occur at any age. If iodine requirements are not met, the thyroid is unable to produce thyroid hormone in sufficient quantities, leading to iodine deficiency disorders (IDD) with associated functional and developmental abnormalities.³

In terms of daily intake of iodine, the World Health Organization (WHO) recommends 90 μg for children aged 0-5 years; 120 μg for children aged 6-12 year; and 150 μg for those over 12 years. Pregnant and lactating women are recommended 250 μg⁴ daily. By ensuring that individuals have an adequate intake of iodine in their diets, IDDs can be prevented.⁵ Although cretinism is the most severe form of iodine deficiency, minor iodine deficiency can also result in reduced intellectual ability, limited work capacity due to mental and neurological impairment.⁴ In 1994, from the 1572 million people globally who suffered from iodine deficiency (28.9% of global population), 11.2 million were affected by overt cretinism, and 43 million people had at least some degree of intellectual impairment.⁵

In Tanzania, the most recent figures indicate that more than 40% of the population live in geographical regions prone to iodine deficiency.⁶ The prevalence of IDD is greatest in the highland regions such as Iringa, Arusha, Mbeya, Rukwa and Ruvuma.⁷ The Tanzania Demographic and Health Survey (TDHS) indicates that such IDD variations occur largely because of differences in the use of adequately iodised salt (15+ ppm), with households in urban areas more likely to use adequately iodised salt (81%) than those in rural areas (51%) such as the highlands.⁷ As recommended by WHO, Iodine Global Network (IGN) and United Nations Children’s Fund (UNICEF), median Urinary Iodine Concentration (UIC) is considered the most practical biochemical marker for the assessment and monitoring of iodine nutrition in the population.⁸ According to the National IDD survey conducted in 2004, about 25% of primary school children in Tanzania had UIC below 100 μg/L.⁶

Universal salt iodization (USI), whereby all the salt used for human consumption is iodized, is the most used intervention to increase iodine intake.⁹ This intervention is widespread, as 68% of households have access to iodized salt.¹⁰ USI is an effective way of delivering iodine to individuals and in term, improving cognition in populations exposed to iodine deficiency.^10,11 USI is also affordable, with annual costs of salt iodization estimated at USD 0.02-0.05 per child, while the costs to prevent child death is estimated at USD 1000. There are also large gains in per Disability-Adjusted Life Years (DALYs), at USD 34-36.¹² Before USI, it was estimated that iodine deficiency leads to losses of USD 35.7 billion in the countries affected, which is significant when compared with an estimated cost of USD 0.5 billion for USI, representing a cost benefit ratio of over 70:1.¹³

In 1995, Tanzania legislation on USI was enacted.¹⁴ This was later revised in 2010, and now, all salt consumed by animals and humans in the country is fortified with iodine. Enforcement of this legislation is challenging in Tanzania, especially in areas where small-scale salt producers operate. As a result, iodized salt is not widely available. Further, although household coverage with iodized salt is above 80% nationally, the coverage of adequately iodized salt is at 47% across Tanzania.¹⁵

Nutritional deficiencies are prevalent among pregnant women in Tanzania, particularly, in areas of low socioeconomic status. However, there is a dearth of information regarding the burden of, and factors associated with iodine deficiency in this population. In 2010 and 2015, the TDHS reported that the prevalence of iodine deficiency amongst pregnant women was 54%, however, the results of the TDHS were heterogeneous across the regions.^16,17 There are also concerns relating to excessive iodine consumption in pregnancy, because although high iodine intakes are well tolerated by most healthy individuals, in some, excess intake can lead to thyroid conditions such as hyperthyroidism, hypothyroidism, and/or thyroid autoimmunity.^18,19 As insufficient and excessive iodine consumption in pregnancy can result in negative health impacts, it is imperative to investigate current iodine levels and to assess how current USI interventions affect iodine intake, especially in highland areas of Tanzania. It is also necessary to set upper as well as lower limits for maternal iodine intake to ensure optimal health outcomes are achieved.¹⁹ Given the above, this study aimed to firstly, determine the likelihood of iodine levels being above or at recommended levels in the urine (μg/L) of pregnant women in their second trimester. Secondly. to assess the likelihood of IDD or otherwise, differing across socioeconomic groups and locations in Tanzania.

Methods

Results

Discussion

This is the first population-based cross-sectional study to assess the magnitude of iodine status and the association with socio-demographic factors and diet in Tanzanian pregnant women. The findings of the study are important since iodine deficiency is the most prevalent micronutrient deficiency, affecting 28.9% of the world population,²⁴ particularly affecting women living in developing countries.²⁵ Iodine deficiency in Tanzania is also high with the most recent figures indicating that more than 40% of the population in the country lives in geographical regions prone to iodine deficiency.⁶ However, this data is largely outdated, as more recent data as well as the most recent efforts to reduce iodine deficiency have focused on primary school children in Tanzania.⁶ Whilst the iodine micronutrient status among pregnant women has been overlooked in recent years.

In this study, pregnant women living in the Mbarali district were more likely to be above the recommended level of UIC [4.09 (95% CI 1.85-9.01)], particularly among individuals aged between 35-49 years [2.51 (95% CI 0.99-6.33)]. Thus, it is important to monitor thyroid function and its associated disorders in this population. Contrary to these results, in 2010, a reanalysis of the Tanzania demographic and health survey reported 54% of pregnant women with iodine deficiency.¹⁶ The discrepancy could be attributed to differences in study methodologies as well as the 10-year lapse between the studies, since significant USI interventions were applied in this period.

In this study a strong association between consumption of freshwater fish and UIC of <150μg/l [odd ratio = 2.60 (95% CI 1.31-5.15)], indicated that pregnant women who consumed fish were at higher risk of iodine deficiency. This finding could be explained by the notion that iodine levels in freshwater fish depend on the locality and the regularity of consumption of fish.^26,27 Moreover, during pregnancy there are variations in the functionality of the thyroid. This can increase the risk of inadequate iodine intake for some mothers. As such, predicting UIC based on usage of iodized salt alone, may not be accurate.^28–30 Other studies have documented that freshwater fish may contain Iodine in levels that can improve daily Iodine intake.²⁶

In countries with successful USI programs, studies have reported an optimal median UIC in pregnant women. As such, USI remains the most cost-effective strategy for achieving reduced IDD.^31,32 However, the full implementation of USI remains a challenge in many sub–Saharan African countries including Tanzania,³³ largely due to the lack of adequate enforcement and, the inadequate monitoring of small-scale salt producers who often do not comply with USI legislation.⁶

This analysis also indicated that pregnant women who had a primary school education were at higher risk of iodine deficiency [odd ratio= 0.29 (95% CI 0.08-0.99)], However, further studies are needed to investigate this association. Excessive iodine intake in pregnant women is also an important area of current research.^34,35 The WHO recommended an increased iodine intake for pregnant women, although evidence is weak.³⁶ However, detrimental effects from more than adequate and excessive iodine intake have been reported in general populations.^37–39 Shi et al. have reported on the associations between UIC and thyroid health among pregnant women and recommend a lower limit for maternal iodine intake during pregnancy than that currently advised by the WHO.⁴⁰ This is also an area in need of further investigation. The question remains if pregnant women in Mbarali district should continue using iodinated salt, and if so at what concentration.

The strength of this study is in its large population-based sample size. The study limitations are as follows, first, the use of UIC to determine individual iodine status could be limited due to the potential for misclassification of participants because of day-to-day variations. Second, UIC reflects recent iodine intake or exposure rather than chronic individual iodine status. Additionally, the use of iodized salt was not assessed in this study. Finally, it would have been useful to have a non-pregnant control group to help ascertain whether lower UI concentrations during pregnancy could be attributed to pregnancy itself or the diet.

Conclusion

The aim of this study was to explore iodine levels in pregnant women living in the Mbeya region of Tanzania. Findings indicate that 17.14% of participants had an insufficient iodine intake whilst, 24.29% had sufficient/adequate urine iodine concentration, and 58.57% had above the recommended levels of iodine. There were differences found between district councils, with the Rungwe DC having the highest percentage (27.9%) of participants with inadequate urine iodine concentrations. Protective factors for IDD included consumption of freshwater fish and having a primary education, whilst factors increasing the risk of excessive iodine intake included being older in age (35-49 years). Due to these findings, this study recommends strategic efforts to ensure that the current USI program addresses the problem of iodine deficiency in pregnant women, and monitor excessive iodine exposure that might have detrimental effects during pregnancy.

Data availability

Author contributions

Conceptualization, TL, RM., AH. SEJ and GHL; project administration and resources, AS, RN, FK and GHL; formal analysis and writing—original draft, TL, RM, AH, SEJ, HAP, AS, RN, FK, ET and GHL; reviewed and edited the manuscript. GB and, RB. All authors: Reviewed and agreed upon the final manuscript.