An investigation of maternal iodine deficiency in the Scottish pregnant population

The thyroid gland utilises iodine for the production of the thyroid hormones thyroxine (T4) and triiodothyronine (T3). During pregnancy demand for iodine increases as the need for maternal T4 increases. A lack of iodine leading to hypothyroidism may be associated with cretinism, mental retardation and neurological and thyroid damage. Maternal hypothyroidism is known to be associated with a lower IQ in offspring. In 2002, a study of 400 pregnant women in Tayside demonstrated a deficiency of iodine intake in 40 % of women. The aim of the current study was to establish the trend in iodine status of the pregnant population over a ten year period, examining the factors in the monitoring of iodine and thyroid hormone levels; and the causes and strategies for correction of iodine deficiency during pregnancy. The ranges and distributions of several thyroid biomarkers were established and the relationships between them and between iodine deficiency and specific adverse pregnancy outcomes were explored. A dietary questionnaire was used to collect information on diet, supplement intake and frequency of consumption of different food types. Retrospective study: Urine and matching blood samples collected between 39 and 112 days of gestation from 3500 women and stored frozen for 10 years were analysed for iodine and thyroid hormone levels. Median urinary iodine excretion (UIE) level in 3326 women with normal pregnancy outcome was 107.00 μg/l: significantly below the WHO recommended median for pregnancy of 150.00 μg/l. Just over 65.0 % of women were categorised as having insufficient dietary iodine intake. These results confirm and extend the findings of the 2002 Dundee study. UIE levels adjusted for creatinine (UCr) to correct for urine concentration variation were also investigated. A matching serum sample was available for thyroid hormone analysis in 3240 (97.0 %) of the urine samples with iodine measurements. Median concentrations of each marker in the retrospective normal pregnancy outcome group were: 1.36 μU/ml (TSH), 13.48 μg/dl (T4), 15.05 pmol/l (fT4), 2.17 ng/ml (T3), 6.13 pmol/l (fT3) and 14.21ng/ml (Tg). Linear regression analysis in the normal pregnancy outcome group showed that geographical area was a significant predictor (p < 0.0001) of UIE and UIE/UCr levels and all 6 thyroid markers. Maternal age was the only other significant predictor of UIE levels. Other significant associations were for UIE/UCr: BMI and maternal age; for TSH: BMI, UIE/UCr, ethnicity and hCG; for T4: maternal age and TSH; for fT4: parity, smoking status and TSH; for T3: BMI, maternal age, parity and TSH; for fT3: BMI and TSH, and for Tg: smoking and TSH. ANOVA testing showed that women who smoked had significantly lower fT4 and Tg (p < 0.0001) than non-smokers. Women from Southeast Asia had lower UIE/UCr and TSH (p = 0.04) and higher T4 and T3 compared with Caucasian women (p < 0.0001). Women in Glasgow tended to have higher UIE, T4 and T3 and lower TSH (p < 0.0001) compared to other geographical areas (p < 0.0001). A subset of 971 serum samples from pregnancies with adverse outcome including pre-eclampsia, small for gestation, extreme or moderate preterm delivery, or a still birth was also available for analysis. Binary Logistic Regression showed that for UIE/UCr, TSH, BMI smoking status and maternal age, the only statistically significant variable was smoking which was associated with a lower incidence of pre-eclampsia and a higher incidence of low birth weight. Prospective study: Urinary iodine excretion (UIE) levels were measured in 1106 pregnant women recruited prospectively. The median UIE value was 81.40 μg/l; - lower than the WHO recommended UIE level of 150.00 μg/l, and lower than the median found in the retrospective study group of 107.00 μg/l. Just over 80.0 % of pregnant women were categorised as having insufficient dietary iodine intake according to WHO criteria. These results point to a decline in iodine intake over a 10 year period. UCr measurements showed that 16.8% of samples had levels below the minimum WHO defined limit of 30 mg/dl. The median TSH concentration value in the matching serum samples was 1.25 μU/ml. Maternal TSH levels correlated positively with newborn TSH levels in patients for whom data was available (p < 0.0001). The median value of fT4 within the prospective pregnancy group was 11.41 pmol/l. A subset of 145 serum samples from the prospective pregnancy group selected using UIE levels as a cut-off parameter had a median Tg value of 12.30 ng/ml. Seventy one percent of participants completed a questionnaire on diet. Sixty eight percent were unaware that iodine was important in their diet during pregnancy and had a lower median UIE (79.87 μg/l) compared to those who were aware of the importance of iodine (86.23 μg/l) although this was not statistically significant. Using UIE, UIE/UCr, nTSH, mTSH, fT4, and Tg in turn as the dependent variable in a general linear regression model with up to 15 different predictors (including intake of fish, meat, yoghurt, cheese, milk, cereals, bread, vitamin supplements, iodinated salt usage and maternal age, smoking status, geographic area and the other biomarkers tested) showed no dietary factors and only geographic area and smoking status as significant predictors for UIE. When UIE/UCr was used as the dependent variable, significant associations were maternal age, and intake of bread, cheese, cereals, and iodine supplements. The UIE/UCr ratio gave more significant associations than UIE alone. For nTSH significant predictors were maternal TSH, geographic area and maternal age. For mTSH significant predictors were smoking status and fT4. For fT4, significant predictors were geographic area, TSH and smoking status. For Birth weight, the only significant predictor was smoking status. For Tg, the only significant predictor was UIE/UCr. Ninety two (11.24 %) participants reported that they were aware of the availability of iodised salt but only 11 (1.22 %) women used it. Drinking water samples were collected from 21 sites across the West of Scotland. Only one water sample from 21 collected had an iodine concentration > 10 μg/l. Conclusion: In summary this study has shown a significant trend of decreasing iodine intake in Scottish pregnant women over the last 10 years