 |
|
Original Articles Indian Pediatrics 2001; 38: 247-255 |
|||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
Biochemical
Assessment of Iodine Deficiency Disorders in Baroda and |
|||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
Key words: Goiter, Goitrogens, Iodine deficiency disorders, Thyroid stimulating hormone. IODINE deficiency disorders (IDD) encompass a variety of health problems caused by insufficient iodine in the diet. Iodine is an essential trace element supplied naturally by food and water for the normal synthesis of thyroid hormones. Normal human health, growth and development, parti-cularly of the brain are directly dependent on thyroid hormone homeostasis(1). The fight against IDD remains one of the major public health challenges at the beginning of the 21st century. The 1999 World Health Organization report estimates that about 2 billion people (38%) are at risk of IDD in 130 countries out of total 191 in the world(21). A combination of mainly clinical and biochemical parameters is used to assess the iodine status of a population. The biochemical indicators comprise urinary excretion of iodine (UI), serum thyroglobulin levels and serum thyrotropin (TSH) levels, whereas the clinical indicators include prevalence measures of thyroid size (goiter) and neuro-logical disability such as cretinism(3). For nearly half a century the benchmark method for determining the state of iodine nutrition has been the measurement of iodine excretion in the urine(4). The method is entirely objective and non-invasive, but has several potential sources of error(5). Blood spot TSH measurement is an excellent indicator for the case detection of hypothyroidism in neonates but may also be used as a surrogate measure of iodine nutrition in the entire community by looking at the skewness of the distribution of the entire set of blood TSH values(4). However, by its very nature it requires that universal neonatal screening be in place, a situation that is not generally found in a developing country like India. Recognition of goiter by palpation is more difficult whereas ultrasonography to measure the thyroid size in schoolchildren is the best prevalence indicator for the assessment of IDD(6). To date, no major survey that uses biochemical prevalence indicators of IDD has been carried out in Gujarat State. However, various epidemiological surveys based mainly on goiter size by palpation have reported Dang as the most severely affected (44%) district for IDD in Gujarat(7). The sale of non-iodized salt has been banned in this district since 1994. The objectives of the present study were: (i) to assess the severity of IDD in Baroda and Dang districts of Gujarat State using bio-chemical prevalence indicators of IDD; and (ii) to establish a biochemical baseline, in a sub-sample of the larger population of Gujarat, that could be used to monitor the effectiveness of iodine replacement programs. Subjects and Methods Population Studies The rural and tribal population of the study group belonged to the low socio-economic stratum of society. Their diet was mainly vegetarian with major consumption of pearl millet (more common in Baroda district), beans, legumes, spices, garlic and onions. The sample comprised 1363 children (aged <1-15 years). Of these, 1121 tribal children were selected randomly by visits to boarding/day schools in various villages (viz., Vaghai, Dediapada, Rutambhara, Baripada, Dungarda and Rambhas) of Dang district. Two hundred and forty-two rural children were surveyed from the villages of Muval and Tentalav in Baroda district by household visits. There were 641 boys and 722 girls (M : F = 0.8). Information on age, sex, height and weight was recorded. Casual urine samples for iodine and blood spots for TSH determination were collected. A sample of 200 specimens would give a relative precision of 20% (50 ± 10% below 100 µg/L). Urinary Iodine (UI) UI analysis used a modified acid-digestion method (method E), based on the reaction between cerium IV and arsenic III (Sandell-Kolthoff Reaction) using a Technicon Auto-analyzer II(8,9). The results were expressed as micrograms of iodine per liter of urine (µg/L). The method does not separate out the interfering substances so they were removed from the urine samples to arrive at true urinary iodine value. Interfering Substances (IS) The urine samples were diluted according to the requirement from the peak and UI was measured as above. The arsenic acid was added to a 200 µL of diluted sample or neat sample (one having low total UI) and kept for one hour and then ceric ammonium sulfate solution was added and the mixture was kept in dark for 24 hours. This was read in Auto-analyzer again and the calculation were done to arrive at true UI and the amount of IS. The standards were also run with the test urine samples and a standard curve was plotted. Blood Spot TSH Commercially available Bioclone ELISA kits for quantitative determination of neonatal thyrotropin (TSH) were used. It is an enzyme-linked immunoassay incorporating a biotiny-lated anti-TSH polyclonal antibody (Antibody reagent) and an anti-TSH monoclonal anti-body bound to the microwell. It is an amplified method, utilizing the biotin-streptavidin linkage to increase the signal generated. The TSH is first eluted from the blood spot and at the same time the eluted TSH binds to the anti-TSH antibody on the microwell. During the next incubation, a ‘sandwich’ is formed between added bio-tinylated antibody, the antibody on the microwell and the eluted sample TSH antigen. The plate is washed to remove unbound material. Streptavidin-perioxidase (Ampli-fication Reagent) is then added and binds to the biotinylated antibody at many sites. The plate is washed again to remove unbound streptavidin-peroxidase, then TMB substrate solution is added. The substrate solution reacts with the enzyme to produce color in direct proportion to the amount of antigen in the sample. From photometric absorbance readings a standard curve is constructed and the TSH in patient sample can be quantified. Anthropometry The Body Surface Area (BSA) was calculated by using the formula(10): BSA(m2) = Weight (kg)0.425 ´ Height (cm)0.725 ´ 71.84 ´ 10–4. Body mass index (BMI) was calculated as: Weight (kg)/Height (m2). Interpretation, Presentation of Results and Availability of Reference Data Biochemical parameters (UI and blood TSH) are meausred on a continuous scale but their results do not have normal Gaussian distribution. The use of means and standard deviations alone may be inappropriate (unless logarithmic transformation of means and standard deviations to get a normal distribu-tion is performed). Hence, presentation of median or other centiles is ideal. Statistical Methods Proportion, mean, standard deviation, median, interquartile range have been used to describe the data as appropriate. Statistical analysis was performed using SPSS version 6.1.2. Results Urinary Iodine Levels The median true UI with the interquartile range (IQR) for all the studied children was 65 (IQR:38-108) µg/L. The median values and IQR for total UI and true UI level for Baroda and Dang district irrespective of sex is shown in Tables I and II, respectively. The 95% lower and upper limits confidence interval estimates of true UI for Baroda and Dang were 87 [79.7, 94.6] and 77 [73.2, 80.4], respectively. Large amounts of inter-fering substances were detected in urine, presumbaly due to goitrogens(6). UI levels were lower in Baroda girls, whilst there was no gender difference in Dang district (Tables I & II).
Classification of the study group into subgroups based on urinary iodine values as recommended by WHO(2) is shown in Fig. 1. In Baroda greater than 78% of girls and 63% of boys were iodine deficient (UI <100 µg/L) while 43% of girls and 27% of boys had moderate iodine deficiency (UI < 50 µl/L). In Dang district 74% of girls and 71% of boys were iodine deficient, and 38% of girls and 40% of boys had UI <50 µg/L (moderate iodine deficiency).
Subjects from all the villages of Dang district except Rutambhara had low amount of interfering substances. Resident girls of Rutambhara boarding school had very high total UI level (60-1080 µg/L), the majority of which was contributed by large amounts of interfering substances (36-816 µg/L) with median true UI of 100 µg/L. Blood Spot TSH Levels The median serum TSH level for the study group was 1.58 (IQR: 0.56-3.2) mU/L whereas mean TSH was 2.08 (SD = ± 2.06) mU/L. Six per cent of the population had whole blood TSH values >5mU/L. Linear regression analysis showed no significant correlation between UI and blood TSH. However, when the population was divided into UI bands, median blood TSH levels were higher in those groups with lower UI level. There was a bimodal pattern of serum TSH distribution in subjects having TSH >5 mU/L (Table III).
The blood TSH levels were significantly different amongst females and males from Dang (p <0.001) but not in Baroda district (p = 0.08) (Tables I and II). The 95% lower and upper limit confidence interval estimate of blood spot TSH for Baroda and Dang was 1.6 [1.3, 1.9] and 2.2 [2.1, 2.4] respectively. The frequency distribution of blood TSH levels for girls and boys of Baroda and Dang districts is shown in Fig. 2. Blood spot TSH values >5 mU/L were seen in 9% girls and 3% boys from Dang and in 3% girls and 4.5% boys from Baroda. TSH values >3 mU/L were noted in 36% of girls and 24% of boys from Dang and in 21% of girls and 13% of boys from Baroda district. There have been no published authoritative normative values for TSH for the studied population or for school children from an iodine replete environment till date.
The distribution of population having TSH >5 mU/L from different villages of Gujarat is shown in Table IV.
Iodine content of salt consumed by majority of the subjects was very low (7-10 PPM), except one family that used salt with iodine content of 2,000 PPM. Drinking water in Dang was lacking in iodine content whereas it was adequate in Baroda district. This is reflected in lower median urinary iodine levels in Dang district. Discussion We used biochemical prevalence indi-cators and epidemiological criteria to assess the severity of IDD in the state of Gujarat aiming at a target population of preschool and school aged children. Using criteria recommended by WHO(3) for defining the severity (prevalence) of IDD as a public health problem, the studied population has mild IDD based on median true urinary iodine and blood TSH levels. It should be understood that "mild" is a relative term; it does not imply that this category of IDD is of little consequence(3). We found that greater than 70% of children remain iodine deficient and greater than 3% of children have blood TSH levels above the normal range of >5mU/L(2). These findings are even more disturbing given that the national iodine deficiency control program has been implemented in this region of India. These findings indicate that further iodine prophylaxis measures and greater monitoring of the effectiveness of such a program need to be undertaken in this region. Although, the study population can be categorized as mild-to-moderate affected by iodine deficiency, there was great variation in the expression of the severity of iodine deficiency amongst individuals and villages. Iodine deficiency was more prevalent and of greater severity in female children from Baroda. In both districts, almost one in four female children had moderately severe iodine deficiency. The increased proportion of girls of child-bearing age having iodine deficiency is particularly important because iodine deficiency in the fetus and infant can lead to irreversible intellectual and neurological damage(11). Although, goiter prevalence rates (GPR) were not measured for this population, we have previously reported that GPR in a similar population are as high as 30% using palpation and almost 100% of children had enlarged thyroid volume for body surface area (>97th percentile of adopted WHO international reference values) using ultrasonography(6). Females from Dang and males from Baroda showed high amounts of interfering substances in the urine probably due to the higher intake of flavonoids in millet, beans, ground nuts and other vascular plants and consumption of spices and organic disulfides in onions and garlic(12). We hypothesize that these interfering substances detected in urine are probable goitrogens or their metabolites. These are known to interfere with synthesis of thyroid hormones and hence are responsible for the increase in blood TSH seen in these subjects. We have identified apigenin in pearl millet from Baroda and its quantity was almost double in one of the village (Tentalav)(6). The higher values for TSH in children from Tentalav may be explained by the greater intake of flavonoids like apigenin, vitexin and glycosyl-vitexin detected in pearl millet(6). There was a strong correlation of median serum TSH values against urinary iodine values, the lower the urinary iodine values the greater the rise in blood TSH values (Table IV). However, there was bimodal pattern of serum TSH distribution in subjects who had TSH >5 mU/L. As expected from the UI data, girls were more likely to have values of blood TSH above 5 mU/L (7.7% of girls, 3% of boys). Coincident ingestion of goitrogens may also have an impact on blood TSH values as seen in the village of Rutambhara. Here, a staggering 18% of the females had serum TSH levels above 5.0 mU/L (Table IV), whilst only 45% and 10% had mild and moderate iodine deficiency, respectively. Biochemical indicators of IDD in this population may underestimate the severity of the problem, in part, explained by the con-comitant impact of goitrogens and malnutri-tion in the pathogenesis of goiter. Besides being the main IDD status indicators, the biochemical parameters are core indicators in monitoring progress towards the goal of eliminating IDD as a significant health problem. The findings of the present study are limited by the relatively small sample size. Nevertheless, given the homogeneity of the population there is little to suggest that these findings are not generalisable. The present study should form the basis for a larger survey in the Gujarat State. In conclusion, based on biochemical prevalence indicators, IDD is a public health problem in Gujarat State. Dang district is more affected due to lack of iodine in drinking water apart from contribution of malnutrition and dietary goitrogens. Iodized salt policy does not appear to have had the desired success in this district. Baroda district is a newly identified pocket of IDD. The expression of IDD in these districts reveals interplay of multiple factors, Contributors: SRB collected and investigated the samples for blood spot TSH and interfering substances. RAF analyzed urine samples for iodine. RMB helped in data collection and drafted the paper. CJE interpreted the data and was responsible for critical revision. SCB coordinated the study, final approval of the version to be published and will act as a guarantor for the paper. Competing interests:
None stated.
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
