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 Table of Contents  
ORIGINAL RESEARCH REPORT
Year : 2019  |  Volume : 16  |  Issue : 1  |  Page : 20-25

Iodine status in pregnant Nigerian women; Does Gestational age matters?


1 Department of Medicine, State Specialist Hospital, Akure, Ondo State, Nigeria
2 Department of Medicine, Endocrinology, Diabetes and Metabolism Unit, College of Medicine, University of Lagos; Department of Medicine, Lagos University Teaching Hospital, Surulere, Idi-Araba, Lagos State, Nigeria
3 Department of Medicine, Lagos University Teaching Hospital, Surulere, Idi-Araba, Lagos State, Nigeria
4 Department of Community Health and Primary Care, College of Medicine, University of Lagos, Idi-Araba, Lagos State, Nigeria

Date of Web Publication14-Feb-2019

Correspondence Address:
Dr. Oluwatosin O Kayode
Department of Medicine, State Specialist Hospital, PMB 603, Akure, Ondo State
Nigeria
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/jcls.jcls_3_18

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  Abstract 


Background: Iodine deficiency affects over 2.2 billion individuals globally. It is the most common cause of hypothyroidism in pregnancy and remains the leading cause of preventable infant intellectual deficits. This study set out to determine the relationship between gestational age and iodine status in Nigerian women. Methods: This was a prospective cross-sectional study with a total study population of 220 pregnant and 110 nonpregnant participants. Urinary Iodine Excretion (UIE) was performed using the Sandell–Kolthoff reaction. Pregnant women were grouped into three trimesters (0–13 weeks, 14–26 weeks, and ≥27 weeks.). Analysis of variance was used in comparison of means, Chi-square test used in analyzing proportions, while P ≤ 0.05 was considered statistically significant. Results: The median UIE was 135 μg/L in pregnant and 120 μg/L in the nonpregnant women. Among the pregnant women, 133 (60.5%) had insufficient iodine intake (UIE <150 μg/L) while 29 (27.3%) of the nonpregnant women had inadequate iodine intake (UIE <100 μg/L). The median UIE was 140, 139, and 120 μg/L in the first, second, and third trimesters, respectively (P = 0.13). The median UIE declined with advancing gestational age. The percentage of pregnant women with inadequate iodine intake was 53.6% in the first trimester and 59% and 72.6% in the second and third trimesters, respectively (P = 0.03). Conclusion: Three-fifths of the pregnant women had inadequate iodine intake. The median UIE decreased with advancing gestation. Iodine supplementation before and during pregnancy would help improve the iodine status in pregnancy.

Keywords: Iodine deficiency, iodine status, pregnancy, thyroid disorder


How to cite this article:
Kayode OO, Odeniyi IA, Olopade OB, Iwuala SO, Odukoya OO, Fasanmade OA. Iodine status in pregnant Nigerian women; Does Gestational age matters?. J Clin Sci 2019;16:20-5

How to cite this URL:
Kayode OO, Odeniyi IA, Olopade OB, Iwuala SO, Odukoya OO, Fasanmade OA. Iodine status in pregnant Nigerian women; Does Gestational age matters?. J Clin Sci [serial online] 2019 [cited 2019 May 27];16:20-5. Available from: http://www.jcsjournal.org/text.asp?2019/16/1/20/252272




  Introduction Top


Global burden estimates suggest that there are 2.2 million cases of iodine deficiency.[1] Developing regions such as South Asia, East Asia Pacific, and Eastern and Southern Africa bear a disproportionate burden of the disease. Women of childbearing age, especially pregnant and lactating women, are considered key populations for diagnosis and treatment of iodine deficiency. Deficiency of iodine, a critical component of thyroid hormones, is responsible for the majority of cases of hypothyroidism. Studies have shown that iodine deficiency is associated with both adverse maternal and fetal outcomes.[2],[3],[4],[5] Untreated overt hypothyroidism in pregnancy increases the risk of maternal hypertension, preeclampsia, placental abruption, postpartum hemorrhage, and cardiac dysfunction.[6],[7],[8],[9],[10],[11] The risks for the fetus or infants include fetal or neonatal prematurity, low birth weight, congenital anomalies, fetal death or stillbirth, intellectual, and neurological deficits.[6],[9],[11] Despite the well-documented maternal and fetal consequences associated with iodine deficiency, data are lacking on the burden of iodine deficiency among pregnant women in Southwestern Nigeria. This study set out to determine the prevalence of iodine deficiency in pregnant Nigerian women residing in Lagos and also the relationship between their iodine status and gestational age.


  Methods Top


This prospective cross-sectional study was carried out in the Lagos University Teaching Hospital (LUTH), a tertiary hospital located in the Southwestern region of Nigeria. This hospital is one of the main referral medical institutions in Lagos state.[12] The hospital comprises 13 clinical departments which include the following: medicine, surgery, pediatrics, obstetrics and gynecology, radiodiagnosis among others. The Obstetrics and Gynecology Department of LUTH runs clinics in the mornings for pregnant and postpartum women four times in a week. Ethical approval was obtained from the Health Research and Ethics Committee of LUTH.

Study participants were selected from pregnant women who attended the obstetrics and gynecology clinics for their antenatal care during the period of study from April to September 2012. The study set out to determine iodine status in pregnant women in Lagos state, and the participants must have resided in Lagos state in the preceding 1 year to be eligible. Second, an obstetric ultrasound scan must have confirmed a singleton pregnancy and gestational age of the pregnancy. The women were selected from a list obtained at each visit from the medical records officer. They were stratified into categories based on their ages and the gestational ages of their pregnancies. The ages of the pregnant women recruited into the study ranged from 21 to 40 years, and they were distributed into these four age categories 21–25, 26–30, 31–35, and 36–40 years. The gestational ages of the pregnant women ranged from 6 to 39 weeks, and they were distributed into these three categories based on their trimesters: 0–13 weeks, 14–26 weeks, and ≥27 weeks. Those who were recruited had an identification mark placed on their case files to avoid their being included in the study again subsequently. The controls were nonpregnant female members of staff of LUTH with similar age to the pregnant women in a ratio 1:2. Pregnancy was excluded if urine β-human chorionic gonadotropin (hCG) pregnancy test was negative. Women with a personal or family history of thyroid dysfunction or diabetes mellitus in first-degree relatives were excluded from the study. All participants gave written informed consent.

Using a 95% confidence interval, a margin of error of 5% and a prevalence of gestational iodine deficiency rate of 14% reported in Nigeria, a minimum sample size of 185 was obtained.[4]

All participants had their demographic information obtained using the questionnaires administered by trained research assistants. Information obtained from each participant included age at last birthday, marital status, and highest level of education. Physical examination was carried out and this included examination of the eyes, neck, hands, legs as well as measurement of weight, height, and blood pressure. Serum thyroid-stimulating hormone (TSH) and free thyroxine (fT4) were quantitatively determined using enzyme-linked immunoassays in 220 pregnant and 110 nonpregnant women. Euthyroid state was defined as TSH levels within 0.22–4.98 μiu/ml and fT4 within 9.56–20.68 pmol/l both obtained from the 2.5th and 97.5th percentile of the values of the control participants. Iodine status is typically assessed using spot urinary iodine measurements.[1] Urinary iodine reflects dietary iodine intake directly because people excrete more than 90% of dietary iodine in the urine.[1] Iodine levels measured in spot urine samples are the recommended indicator by the WHO for assessing iodine status in populations. The WHO standard operating procedure for analyzing iodine levels was strictly adhered to.[1] Samples of urine were obtained from the women for urinary iodine estimation using the method of Sandell–Kolthoff reaction.[1] Urine was first digested with ammonium persulfate before carrying out this reaction, to rid the urine of interfering contaminants. The iodine in the urine samples catalyzed the reduction of ceric ammonium sulfate (yellow color) to the cerous form (colorless) in the presence of arsenious acid. The degree of reduction in color intensity of the yellow ceric ammonium sulfate was proportional to the iodine content in the urine sample.[1]

The WHO epidemiological criteria for assessing iodine status based on urinary iodine concentrations were used. Adequate iodine nutrition status based on urinary iodine concentrations for pregnant women ranged between 150 and 249 μg/L, insufficient levels were <150 μg/L, and above requirements were 250–499 μg/L.[5] For nonpregnant women, adequate iodine nutrition assessed using urinary iodine concentrations ranged between 100 and 199 μg/L, insufficient levels were <100 μg/L, and more than adequate levels were ≥200 μg/L.[5]

To assure the quality of data derived from this study, precision analysis was conducted for all assays which were TSH, free T4 (FT4), and urinary iodine concentrations. For each analyte, a sample each at three different levels of concentration (low, medium, and high) is measured several times and the coefficient of variation (CV) calculated. The mean concentrations of the samples at the three different levels of concentration were measured. The CV was calculated using the formula (standard deviation/mean) ×100 (intraassay). The interassay CV was also measured.

Standard acceptable coefficients of variation for intraassay and interassay precision were used to determine the precision of the study assays. CV <15% for intraassay and interassay precision was acceptable.[13] The hormonal assay in this study was done in duplicate. This was to ensure accuracy as much as possible.

Data analysis was done using the Statistical Package for the Social Science version 17th edition (IBM corporation). Analysis of variance was used in comparison of means, Chi-square test was used in analyzing proportions, and Spearman's correlation was used to determine the relationship between urine iodine excretion levels with gestational age. P ≤ 0.05 was considered to be statistically significant.


  Results Top


Two hundred and twenty pregnant and 110 nonpregnant women were studied [Table 1]. The mean gestational age of all pregnant women was 20.6 ± 9.6 weeks ranging from 7 to 39 weeks with a median age of 19 weeks. About 70% of the participants were multigravida. The mean ages of both groups of women were similar 30.6 ± 5.2 and 30.4 ± 6.0 years (P = 0.7).
Table 1: Distribution of the study participants by chronological and gestational ages

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The coefficients of variation for elevated, medium, and low urine iodine excretion concentrations for both intra- and interassays were within acceptable limits [Table 2].
Table 2: Precision of urine iodine excretion

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Thyroid disorders were observed in 13 (5.9%) pregnant women. Seven out of these 13 women 53.8% had hypothyroidism while 46.2% had hyperthyroidism. Four out of the seven women with hypothyroidism had the overt form while three (42.9%) of the women had subclinical forms. Of the four women with overt hypothyroidism, three were in their third trimester while the fourth woman was in her second trimester. Two out of the three women with subclinical hyperthyroidism were in their first trimester, while the third woman was in her second trimester. In contrast, 2 out of 110 (1.9%) of the controls had subclinical hypothyroidism, while another two (1.9%) had subclinical hyperthyroidism. There was statistically significant difference in thyroid status when comparing both groups P = 0.02.

The mean TSH levels of 2.7 ± 0.58 μiu/ml in pregnant women were significantly higher than 2.59 ± 0.43 μiu/ml in nonpregnant participants (P = 0.01). The mean TSH levels were 2.42 μiu/ml in the first trimester and 2.75 μiu/ml and 2.9 μiu/ml in the second and third trimesters, respectively (P = 0.01). The mean FT4 levels of 14.2 ± 1.71 pmol/L in pregnant women were significantly lower than the value of 14.90 ± 3.3 pmol/L in nonpregnant participants (P = 0.01).

The urine iodine excretion (UIE) values ranged from 40 to 225 μg/L in pregnant women and 57–210 μg/L in the nonpregnant women. The median UIE was 135 μg/L in pregnant women and 120 μg/L in the nonpregnant women [Table 3].
Table 3: Urinary iodine concentration and distribution of nutritional iodine status across the three trimesters

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Of 220 pregnant women, 87 (39.5%) had optimal iodine nutrition while 133 (60.5%) had insufficient iodine intake. In contrast, 29 (26.4%) of the 110 nonpregnant women had mild iodine deficiency and 81 (73.6%) had optimal iodine nutrition [Table 3]. There was a statistically significant difference in the iodine status of both groups P = 0.027.

The median iodine excretion rates were 140, 139, and 120 μg/L in the first, second, and third trimesters, respectively [Table 3]. The result of UIE of the 69 pregnant women in their first trimester showed that 32 (46.4%) had optimal iodine nutrition while 37 (53.6%) had insufficient iodine intake [Table 3]. Among the 78 pregnant women assessed in the second trimester, 46 (59%) had inadequate iodine intake while 32 (41%) had optimal iodine nutrition. Of the 73 pregnant women in their third trimester, 20 (27.4%) had adequate iodine nutrition while 53 (72.6%) had insufficient intake.

The percentage of pregnant women with inadequate iodine intake was 53.6% in the first trimester and 59% and 72.6% in the second and third trimesters, respectively (P = 0.03) as shown in [Table 3]. Correlation analysis performed showed that there was an insignificant negative correlation between urinary iodine excretion and gestational age (Spearman's rho correlation coefficient: 0.112; P = 0.1). With increasing gestational age, urinary iodine excretion levels decreased.


  Discussion Top


Our data demonstrate that thyroid dysfunction including both hyper and hypothyroidism is higher in pregnant women 5.9% compared to nonpregnant women 3.6%. El-Bashir et al. in 2015 had similar findings in Northwestern Nigeria where thyroid disorders among pregnant women and controls were 5.3% and 5%, respectively.[14] Thevarajah et al. in Malaysia,[15] Pasupathi et al. in India,[16] Zarghami et al.[17] in Iran, and El-Bashir et al. in Zaria, Nigeria,[14] all reported increasing TSH levels and declining fT4 levels with increasing gestational age. It has been shown during pregnancy that changes in albumin and free fatty acid concentrations affect the binding of T4 and T3 to carrier proteins, and this lowers the blood levels of FT4 and FT3 as pregnancy progresses and in turn, leads to an increase in TSH levels.[17],[18]

The likely reason for the higher frequency of hypothyroidism in pregnancy as compared to the nonpregnant women is that during pregnancy, there is increased thyroid hormone production and increased fetal iodine requirements. Consequently, dietary iodine requirements are higher in pregnancy than they are for nonpregnant adults. In women with inadequate iodine intake before and during pregnancy, maternal hypothyroidism could occur as increased demand of pregnancy outstrips supply.[19],[20]

The finding of overt hypothyroidism of 1.8% in this study was similar to 1.6% in normotensive pregnant women in a study conducted by Abdulsalam and Yahaya in Kano, Nigeria, in 2015.[21] This figure is comparable to 2.2%–2.5% reported in European and American pregnant women[7],[8],[10] but lower than 3.2% reported in Tunisian women.[22] The reason for differences in the prevalence of hypothyroidism seen in study populations could be as a result of the differences in the amounts of iodine in soil, water in the different regions, and the amount of iodine contained in food consumed by the study participants.[5] Ethnic disparities also result in geographic variability of hormonal values.

Majority of the pregnant women with subclinical hyperthyroidism were in their first trimester. This was a similar finding among the Tunisian pregnant women and could be due to the direct stimulatory effect of placental hCG on thyrocytes.[22] This could induce a small and transient increase in fT4 levels, and in turn, a partial TSH suppression near the end of the first trimester when circulating hCG is at peak levels.

In this study, 26.4% of the nonpregnant women had insufficient iodine intake, 17.9% was reported by Sebotsa et al. in 2005 in Lesotho, while Watts et al. in Malawi 2015 reported a lower proportion of their women (12%) with iodine deficiency.[23],[24] This may be due to differences in the iodine content of locally sourced foods, water, or salt. The median UIC in the populations of nonpregnant women in the three studies was adequate and thus reproduces the 2016 Global Scorecard of Iodine Nutrition findings for Malawi, Nigeria, and South Africa as having adequate iodine nutrition in school-age children.[25]

In Tanzania's 2010 Demographic Survey, pregnant women had a median UIE of 134 μg/L which was lower than the 194 μg/L obtained in controls.[26] Similarly, El-Bashir et al. in 2015 in Northwestern Nigeria observed that nonpregnant women had a better iodine status than the pregnant women. In their study, the median UICs were 205 and 193 μg/L for the control participants and pregnant women, respectively.[14] In this study, although the median UIE of 135 μg/L in the pregnant women was higher than 120 μg/L in the nonpregnant women, the reference range of UIE in pregnant women was higher and indicated a relatively higher iodine nutrition insufficiency in the pregnant women.

The median UIE for the pregnant women in this study is lower than UIE stated in Orlu, Imo state, and Zaria, Kaduna state, but higher than reported values in Ohafia, Abia state.[4],[14],[23],[27] These findings in different geographical areas of Nigeria could be accounted for by inadequate dietary diversifications resulting in mineral and vitamin deficiencies in the differing study populations.

About three-fifths of the pregnant women in this study had inadequate iodine nutrition. Simpong et al. in Ghana in 2016 reported a prevalence of iodine deficiency in pregnant women of 42.5% and this was attributed to noncompliance with the use of iodized salt.[28],[29] Ersino et al. in Southern Ethiopia in 2013, Kedir et al. in Eastern Ethiopia 2014, and Zenebe et al. in Southwestern Ethiopia in 2014 found that 82.8%, 80%, and 88.9%, respectively, of the pregnant women had insufficient iodine intake.[30],[31],[32] The iodine deficiency states were said to be as a result of poor nutritional status and lack of iodinated salt supplementation. A study conducted in Southeastern Nigeria in 2011 by Igwe et al. revealed that none of the pregnant women had a UIE of <l50 μg/l which indicates 100% insufficient iodine intake while in Zaria, it was reported that 46% of pregnant women had UIC of <150 μg/L. Although Lagos which is in Southwest Nigeria is an iodine-replete area, it has been reported in an iodine-replete area that although it appeared that the general population had adequate iodine intake, some pregnant women had UIEs below the recommended levels and were at risk of iodine deficiency.[33],[34]

Three-fifths of the pregnant women had inadequate iodine nutrition, while Ujuwondu et al. in Southeast Nigeria reported in 2010 that < one-fifth of the women had inadequate iodine nutrition. The reference range of UIE in pregnant women used by Ujuwondu et al. was <l00 μg/l.[4] This is lower than what was used in this study and this is likely to account for this difference.

The proportions of women with inadequate iodine nutrition are positively related to increasing gestational age which was demonstrated by Ujuwondu et al. in Imo state and El-Bashir et al. in Kaduna state Nigeria. A similar trend was also reported by Fuse et al. in Tokyo and Brander et al. in Switzerland.[4],[35],[36] This decrease could be attributed to the increased demand of iodine as a result of pregnancy.


  Conclusion Top


Three-fifths of the pregnant women had inadequate iodine intake. The median UIE decreased with advancing gestation.

Recommendation

The findings from this study show the need to improve the iodine status of pregnant women. It is important that health-care providers, especially those ones who take care of women in the reproductive age group counsel women on the importance of adequate iodine intake, dietary diversity, consumption of foods with higher iodine content such as leafy vegetables and fish, use of iodized salt in cooking, and to encourage iodine supplementation during pregnancy. Public health strategies should include dissemination of accurate information and nutritional education to the general population, research to encourage better understanding of the nutritional status of locally produced foods, universal iodization of salt, and regular monitoring of its implementation.

Limitations

A limitation of this study was the use of a single casual urine sample to assess iodine status. Despite being the method accepted by the International Committee for the Control of Iodine Deficiency Disorders/UN Children's Fund/WHO (2001) for the assessment of the iodine status of groups of people, intraindividual day-to-day variation in iodine intake occurs. The validity of multiple spot urinary collections compared with a single urine sample for the assessment of iodine status in pregnancy warrants further research.

No questionnaire estimates of iodine intake from diet, use of iodized salt, or intake of vitamin/mineral preparations containing iodine were documented.

Further studies are also required to determine the effects of thyroid dysfunction and iodine deficiency in offspring of pregnant women.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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