The study used data from the nationally representative 2010 Tanzania Demographic and Health Survey (TDHS). The survey maintained all the protocols prescribed by the World Health Organization . The survey was designed to provide data to monitor the population and health situation in Tanzania as a follow up to the previous four TDHS conducted during 1991–2005. The survey utilized a multistage cluster sample based on the 2002 Tanzania Census and was designed to produce separate estimates for key indicators for each of the eight geographic zones of the country: Western (Tabora, Shinyanga, Kigoma), Northern (Kilimanjaro, Tanga, Arusha, Manyara), Central (Dodoma, Singida), Southern Highlands (Mbeya, Iringa, Rukwa), Lake (Kagera, Mwanza, Mara), Eastern (Dar es Salaam, Pwani, Morogoro), Southern (Lindi, Mtwara, Ruvuma), and Zanzibar (Unguja North, Unguja South, Town West, Pemba North, Pemba South). Data collection began on 19 December 2009 and was completed on 23 May 2010.
The survey obtained detailed information on household characteristics, fertility levels and preferences, awareness and use of family planning methods, childhood and adult mortality, maternal and child health, breastfeeding practices, antenatal care, childhood immunization and diseases, nutritional status of young children and women, malaria prevention and treatment, women’s status, female circumcision, sexual activity, knowledge and behavior regarding HIV/AIDS, and prevalence of domestic violence. The survey collected information of a weighted sample of 11,224 women aged 15–49 years. The sample was weighted by the weighting factor provided in the survey data. Of the women, 10,260 were currently not pregnant and 17 had missing information regarding urinary iodine concentration (UIC). Thus a total of 10,277 women aged 15–49 years were excluded from the analyses leading to a sample size of 947 women. The details of the survey are well-described in the 2010 TDHS report .
The unit of analysis of this study was ‘subclinical iodine deficiency’ which was measured from UIC. WHO recommends urinary iodine concentrations of <150, 150–249, 250–499, and ≥500 μg/L to indicate insufficient, adequate, more than adequate, and excessive levels of iodine intake respectively in the pregnant women population . In this study, UIC <150 μg/L defined subclinical iodine deficiency and ≥150 μg/L defined absence of subclinical iodine deficiency. The group with <150 μg/L was further categorized into <20 μg/L (severe iodine deficiency), 20–49 μg/L (moderate iodine deficiency), and 50–149 μg/L (mild iodine deficiency) .
Previous studies revealed that a number of socio-demographic, cultural, political and environmental factors affect women’s iodine nutritional status. Household salt is an important factor in analyzing iodine nutritional status. In this study, household salt was classified as “iodized” and “not iodized”. The current age of the study women was classified into three categories: 15–24, 25–34 and 35–49 years. Since women’s education is an important factor for nutritional status, it was broken down into three categories: no formal education, primary education, and secondary or higher education. One of the background characteristics used in this study was the household economic status namely ‘wealth index’. The wealth index used in this study was developed and tested in a large number of countries to measure inequalities in household income, use of health services, and health outcomes . It is an indicator of the level of wealth that is consistent with expenditure and income measures . The wealth index was constructed from data on household assets, including ownership of durable goods (such as televisions and bicycles) and dwelling characteristics (such as source of drinking water, sanitation facilities, and construction materials). To create the wealth index, each asset was assigned a weight (factor score) generated through principal component analysis, and the resulting asset scores were standardized in relation to a normal distribution with a mean of zero and standard deviation of one . Each household was then assigned a score for each asset, and the scores were summed for each household; individuals were ranked according to the total score of the household in which they resided. The sample was then divided into quintiles from one (poorest) to five (richest). The eight geographic zones of the country – Western, Northern, Central, Southern Highlands, Lake, Eastern, Southern and Zanzibar –were selected as another background variable in this study to assess zonal variation in iodine deficiency status of pregnant women. The family size was grouped as a dichotomous variable namely ‘<5 persons’ and ‘≥5 persons’.
To examine the relationship between subclinical iodine deficiency status and background characteristics of the women, UIC was made a binary response. If a woman was iodine deficient, she was coded as ‘1’ and coded ‘0’ otherwise. In doing so, the women with more than adequate, and excessive levels of iodine intake were coded ‘0’. Fixed effect logistic regression analyses were used in this study. Simple logistic regression model was applied to examine the association between background characteristics and subclinical iodine deficiency while a multivariable logistic regression model was applied to assess the net effects of the selected factors on ‘subclinical iodine deficiency’ among pregnant women. The results of the regression analyses are presented by odds ratios (OR) with 95% confidence interval (CI) for easy understanding of the effect of the corresponding factor after controlling for other confounders. Statistical analyses were performed using SAS version 9.4.
Ethics approval was granted to the National Bureau of Statistics (NBS) by the Tanzania’s National Institute for Medical Research, the Zanzibar Medical Research and Ethics Committee, the Institutional Review Board of ICF International, and the US Centers for Disease Control and Prevention. Informed consent was obtained individually from all respondents before the start of the interview. Permission to re-analyze and publish the findings was granted by the NBS.