Socio-economic and spatial correlates of subclinical iodine deficiency among pregnant women age 15–49 years in Tanzania

In the past decade there has been a great focus on increasing iodine intake, but iodine deficiency remains a major health problem around the world. In the year 2011, it was estimated that about 2 billion people in the world had inadequate iodine intake [1]. Iodine deficiency particularly impacts young children and pregnant women especially in developing countries. About 38 million young children suffer from iodine deficiency in developing countries every year [2–4] and national surveys suggest that about 20–65% of pregnant women are iodine deficient in developing countries [5–8].

Iodine deficiency has severe effects on pregnant women and newborns [9, 10]. These include death to women during childbirth, miscarriage, stillbirth, congenital abnormalities and intellectual dysfunction [11, 12]. Iodine deficiency may also lead to brain damage and poor growth in newborns leading to lifelong negative effects on cognitive development and learning abilities [2, 11, 12].

A 1999 report by WHO and UNICEF indicated that 44 out of 46 sub-Saharan African countries, including Tanzania, had high prevalence of iodine deficiency [13]. A study done in Ethiopia in 2014 has shown that overall median UIC in pregnant women was 58.1μg/L and 82.8% of the women had subclinical iodine deficiency [14]. Another study conducted in Congo in 2013 revealed that the overall median UIC in pregnant women was 138 (inter quartile range: 105–172) mg/l, indicating iodine deficiency [15]. In Tanzania it was estimated that 41% of the population lived in areas exposed to iodine deficiency were iodine deficient [16]. Despite of iodine intake improvement made s in Tanzania following the implementation of universal salt iodization (USI) in 1985 when all salt for human consumption was fortified with iodine [17]; many still lack access to adequately iodized salt, and prevalence of iodine deficiency remains high especially among pregnant women [18]. Studies on iodine deficiency have been conducted in Tanzania, but most of the studies focused on the general population, young children and women aged 15–49 without giving special consideration to only pregnant women [16, 19–23].

A number of studies have been conducted in Western countries such as France [24] and Kosovo [25] as well as in Mali [26] and Bangladesh [27] to establish the association between women’s iodine nutritional status and several socio-economic characteristics. These studies found a significant association between women’s iodine nutritional status and demographic characteristics such as wealth, socio-economic status, and education and modern contraceptive use though results were mixed on the relationship between age and iodine nutritional status. In Kosovo, the relationship between age and iodine nutritional status was found to be not significant, whereas in studies conducted in France, Mali, and Bangladesh, age and iodine nutritional status were found to be significantly associated.

Thus recent studies on the association between women’s socio-economic characteristics with their iodine nutritional status in pregnant women aged between 15 and 49 years have had wide coverage but are limited in Tanzania [28]. Therefore, there is a need for further investigation into factors that influence iodine deficiency. The present study aimed to determine the association between socio-demographic factors and subclinical iodine deficiency among pregnant women aged between 15 and 49 years. This study will enrich the availability of information and contribute to developing appropriate interventions for reducing iodine deficiency among pregnant women in Tanzania.

Our analysis has shown that the prevalence of iodine deficiency among the surveyed pregnant women was unacceptably high. The existence of a large proportion of pregnant women presenting with subclinical iodine deficiency is indicative of inadequate access to properly iodized salt. Though USI policy in Tanzania passed in 1985 required that all salt in the country be iodized, there have been challenges to full implementation. There is a vast and complex network of salt producers in Tanzania each with different distribution schemes and different access to salt iodization technology. Additionally, there has not been consistent monitoring of iodized salt quality, and as a result, people still suffer from iodine deficiency due to consumption of salt that is insufficiently iodized and inability to access properly iodized salt as well as inability to afford iodized salt and lack of education about iodized salt [36]. Inadequate dietary iodine intake as a result of these factors is particularly dangerous in pregnant women as it increases the risk for unwanted pregnancy outcomes. Children born to iodine deficient mothers are more likely to suffer from hypothyroidism, impaired neurological development, and, in severe cases, cretinism [37].

Recent population studies show that women across the globe are at increased risk of developing iodine deficiency especially during the pregnancy period when iodine requirements are increased at least 50% [38, 39]. Though global estimates are not available, national level studies have been used to estimate the prevalence of iodine deficiency in pregnant women. A study in Kolkata, India found that 37% of pregnant women had insufficient iodine status [38]. Similarly, a higher prevalence of iodine deficiency among pregnant women was reported in Belgium, a mildly-iodine deficient country, where one in six pregnant women were iodine deficient indicating that the pregnancy period makes women especially vulnerable to insufficient iodine intake [7].

In this study, the pregnant women aged 25–34 years had higher odds of presenting with subclinical iodine deficiency. The 25–34 age group represents the most active reproductive age group and the age group during which the most women are pregnant, a time when iodine requirements are significantly greater, or lactating, when much maternal iodine is passed from mother to infant during breastfeeding. The demands of pregnancy and lactation may make the 25–34 age group particularly vulnerable to iodine deficiency. A study in Bangladesh showed that pregnant and lactating women had a higher prevalence of iodine deficiency (37 and 33% respectively) compared to their non-pregnant counterparts (3%) [40]. Similarly, our study has shown that women having one to four children were at more risk to iodine deficiency compared to the ones who have not given any delivery. Thus if left unaddressed, iodine deficiency in this age group may have serious health implications to both the mother and the newly born child [7].

Women belonging to the poorer and poorest wealth index categories had higher odds of presenting subclinical iodine deficiency. In addition, wealth index was associated with both consumption of meat and fish. The consumption of meat and fish in most cases is associated with improved income thus indicates that the poor did not have adequate access to iodine rich foods such as fish which are also considered expensive. Contrary to our expectation, pregnant women who belonged to the poorest wealth category had lower odds of having iodine deficiency compared to the poorer wealth category. The observed difference among poorest and poorer pregnant women suggests a need for in-depth analysis to understand the driving factors that contribute to the observed difference between the two wealth categories.

Consumption of adequately iodized salt in the household was beneficial at reducing the risk of developing subclinical iodine deficiency. The existence of few pregnant women who consumed adequately iodized salt presenting subclinical iodine deficiency highlights the importance of consuming iodized salt in the household and calls for more efforts to 1) ensure that salt quality is standardized to contain the recommended levels of iodine and 2) ensure that households have equal access to properly iodized salt. Finally, a geographical disparity in the prevalence of iodine deficiency was observed among pregnant women. In the Southern zone, the odds of iodine deficiency were significantly higher compared to the Lake zone which may result from the large number of small-scale salt producers in this region that are not closely regulated and produce salt with highly variable iodine content. In a survey of small-scale salt producers, 93% did not have adequate iodine content in their salt [41]. Furthermore, in an attempt to standardize salt content, some salt producers, both small- and large-scale, were provided with salt iodation machines, but investigators found that most salt factories had stopped using them due to high operating costs, inability to repair machines when broken down, and in effective technical support for the machines. Instead, these factories used manual hand spray pumps to iodize the salt yielding highly variable, and oftentimes less than recommended, iodine content in the salt [41]. The Eastern zone, which is also classified as being relatively food rich and economically better off, had the lowest odds of having a pregnant woman presenting with subclinical iodine deficiency. This region may also suffer the consequences of inadequately iodized salt from small-scale salt producers though the presence of large- and medium-scale salt producers may counteract this fact. Additionally, the Eastern zone’s proximity to the coast may allow for greater access to and consumption of fish and sea vegetables that are rich in iodine. However, the relationship between proximity to the coast and iodine deficiency requires further investigation as the Southern zone also has much coastline but has a significantly higher odds of iodine deficiency. The Northern zone had a lower odds of iodine deficiency than the Lake zone. This may be because of concentrated areas of tourism in this zone (i.e. Mount Kilimanjaro, Serengeti, Ngorongoro Crater, Lake Manyara, and Tarangire National Park) and tourism’s impact on health and development. For example, tourism brings employment opportunities and thus increased income to the local people which permeates the public sector, businesses and private households. Likewise, tourism may also bring competition and lower the price of household commodities such as salt [42]. Thus the various consequences of tourism could contribute to reduced iodine deficiency in the local population though the health effects of tourism are numerous and complex and require further study.

The overall prevalence of iodine deficiency among pregnant women in Tanzania was 54%. Among the set of independent variables included in this study, women age, socio-economic status, iodine content in household salt, and residing in certain geographic zones were significantly important predictors of iodine deficiency in the study population. Continued efforts towards full implementation of universal salt iodization in Tanzania may be an effective strategy for tackling iodine deficiency.