Study design and setting
A school-based cross-sectional study was conducted in May 2016, in Dabat District, northwest Ethiopia. The district is 821 km from Addis Ababa, the capital city of Ethiopia. It has 26 rural and four urban Kebeles (smallest administrative unit in Ethiopia) with a total population of 175,737. The altitude of the district ranges from 1000 to 2500 m above the sea level. Cereals, such as maize, sorghum, wheat, and barley are the main staple crops cultivated in the district. The study was conducted at Dabat Health and Demographic Surveillance System (HDSS) site which covers 13 randomly selected kebeles (4 urban and 9 rural) and stratified by ecological zones as highland, middle land and lowland [18, 28, 29].
Study participants
All school children aged 6–12 years who lived in the HDSS site and attending school during the study period were eligible for the study. The sample size of the study was calculated using Open Epi version 2.3 with the following assumptions: 29.1% prevalence of goiter in school aged children in Dabat district [18], 95% Confidence Level, a 10% non-response rate and a 5% margin of error. The minimum sample size of 349 was estimated.
Participants’ recruitment
A multistage cluster sampling technique was employed to select participants in order to recruit a representative population sample. Initially, schools within the HDSS site were stratified into urban and rural based on their geographic location. Then, five rural and one urban schools from each HDSS site were randomly selected to be included in the survey by lottery method. List of all students aged 6–12 years and enrolled in the schools were identified from the schools’ registry. All Students aged 6–12 in the included schools were randomly selected to take part in the survey using a systematic random sampling technique. The total number of students selected from each school was proportionate-to-student size of the schools.
Data collection
Physical examination of goiter and collection of urine samples from children aged 6–12 years were conducted at schools. Information on child’s name and home address were recorded and used to reach the parents of children. Questionnaire was used to collect the socioeconomic and nutrition related information in children and their parents. The pilot testing of the Questionnaire was done on 22 school aged children and their parents in the district who were not part of the study. Some wordings of the Questionnaire were adapted to the local context and total amount of time required for interview and child assessments defined following the pilot test.
Assesement of urinary iodine concentration
A 10 ml spot urine sample was collected from each child using disposable plastic cup and immediately transferred to cap polyethylene test tube and labeled with an identification number. The urine samples were stored in cold chain until the end of the day and then it were stored at below -20oc refrigerator until transported to Ethiopian Public Health Institute, a national accredited laboratory, for analysis. The UIC was analyzed using the Sandell Kolthoff reaction [26] method recommended by WHO/ICCIDD/UNICEF. Iodine status was classified according to WHO in which iodine deficiency was defined as UIC < 100 μg/l, adequacy at 100–199 μg/l, above requirement at 200–299 μg/l and iodine excess at > 300 μg/l [26].
Assessment of goiter
Goiter was assessed by a qualified health officer by inspection and palpation. Training on the techniques used to perform thyroid examination and the WHO grading of goiter was given for the assessors before the actual data collection. The physical examination was performed following techniques recommended by the WHO [26]. Goiter was defined as; grade 0 if it is not palpable or visible, grade 1 if it is palpable but not visible and grade 2 if it is visible and palpable [26].
Assessment of household salt iodine content
A tablespoon of salt was collected from each household and was kept in a dark, dry and cool place to inhibit iodine evaporation. The MBI international Rapid Test Kit (RTK) obtained from UNICEF Ethiopia were used to measure iodine content of the salt semi-quantitatively [26]. Using this method, level of iodine in the salt was classified as 0 ppm (PPM), < 15 PPM and ≥ 15 PPM. A level of iodine ≥15 ppm was categorized as adequate iodine utilization [26]. The coverage of iodized salt use was calculated as the proportion of households having salt iodine content > 0 ppm among all households included in the study.
Assessment of socio-economic status (SES)
A structured interviewer-administered questionnaire was used to collect data on SES of parents 1 week before assessment of goiter and collection of urine sample. Data were collected by health extension workers at households by asking about the sociodemographic characteristics, wealth index, behavioral characteristics and dietary intake/pattern of parents and the child. Questions assessing the household wealth index and child’s diet diversity score were taken from the Ethiopian Demographic and Health Survey (EDHS) questionnaire [18, 30]. Principal Component Analysis (PCA) was used to assess the wealth index from household assets. Variables related to the size of agricultural land, the amount of grains harvested, house types and the number of livestock were asked to calculate the wealth index. Then, a code between 0 and 1 given to the variables and analyzed using the PCA. The factors were ranked in to the lower, medium and higher tertiles. Finally, these tertiles were categorised as rich, medium and poor. Food frequency questionnaire was used to collect the food intake of school children in the preceding day of the survey.
Dietary assessment
Intake of foods from the seven food groups which include starchy staples, organ meat, flesh meat and fish, egg, green leafy vegetables, vitamin A rich fruits and vegetables, other fruits and vegetables were recorded to the dietary recall questionnaire by asking the Mothers about the types of foods eaten by the child in the preceding full day of the survey. Dietary diversity score (DDS) was calculated from the one-day (24 h) dietary intake data. The dietary recall questionnaire was validated and has been applied to determine dietary diversity score in Ethiopia [30]. The minimum DDS which is optimal for child health and development is four, hence children scoring bellow four were categorized as having poor DDS [13, 31]. The dietary history on major food sources of iodine including milk, meat, egg and some goitrogenic foods including cabbage and millet were recorded. The source of drinking water for the households was also collected to observe the degree of iodine deficiency across the source of drinking water. Nine yes/no questions on mothers’ knowledge regarding iodine deficiency disorders, benefits of iodine intake, food sources of iodine and proper utilization of iodized salt has been asked [14, 18]. The questions were coded between 0 and 1 and analyzed using PCA to produce factor scores. The factors summed up and ranked to a higher and lower tertiles. Finally, Mothers’ knowledge was categorized as poor or good.
Data management and statistical analysis
Data were checked and entered into EPI-Info version 7 and exported to SPSS version 20 statistical software for further analysis. The UIC data were not normally distributed and were log-transformed for analysis. The kappa coefficient (k) was calculated to assess the agreement in defining iodine status between goiter and UIC. The strength of agreement was categorized as poor for k value 0.0–0.20, fair for k value 0.21–0.40, moderate for k value 0.41–0.60, good for k value 0.61–0.80, and excellent for K value 0.81–1.00 [32]. One-way Analysis of Variance (ANOVA) was used to compare mean of log-transformed UIC values among key variables which include place of residence, maternal educational status, wealth index, salt iodine content, presence of goiter, and child’s diet score. Significant was set at a P-value less than 0.05.