Abstract: Background and Objective: Pregnancy exacerbates iodine deficiency and therefore increases the risk of subclinical hypothyroidism (SCH). This study examines the links between iodine deficiency in pregnancy and subclinical hypothyroidism on one hand and preeclampsia, oxidative stress and carotid intima-media thickness on the other hand. Materials and Methods: This case-control study involved 200 participants with preeclampsia. The controls were 150 age-matched pregnant women without preeclampsia with complete data. Carotid intima-media thickness (cIMT), nitric oxide (NO), urinary iodine concentration (UIC), serum thyroid-stimulating hormone (TSH), thyroxine (T4) and triiodothyronine (T3) were measured and compared across controls, mild preeclampsia cases and severe preeclampsia/eclampsia cases using one-way ANOVA. A p<0.05 was considered statistically significant. All analyses were performed using the SPSS for windows version 23.0. Results: There was a significant negative association along a biologic gradient of mean UIC and NO: Controls, cases with mild preeclampsia and cases with severe preeclampsia/eclampsia (p<0.0001). However, there was a significant positive association along a biologic gradient of mean TSH, T3, T4 and cIMT: Controls and cIMT cases with mild preeclampsia and cases with severe preeclampsia/eclampsia (p<0.0001). Conclusion: Iodine deficiency and its associated pro-oxidant subclinical dysthyroidism is not only a risk factor for preeclampsia with elevated cIMT vascular stiffness and oxidative imbalance related to TSH and NO but also for future cardiovascular diseases among women of reproductive age.
INTRODUCTION
Iodine deficiency is one of the leading causes of hypothyroidism and affects about two billion people globally1. Even in countries such as the USA, Australia and Western Europe, once thought to be iodine replete, various population groups have recently been found to have mild to moderate iodine deficiency, especially children and pregnant women. Extreme manifestations of iodine deficiency such as cretinism and endemic goitre are becoming increasingly rare due to salt iodization2. However, chronic mild to moderate iodine deficiency is associated with a subtle impairment of intellectual ability and work output2 and could predispose to subclinical hypothyroidism (SCH), in which clinical manifestations of hypothyroidism are mild. It occurs in about 30% of patients.
Patients with SCH have elevated TSH coupled with normal FT3 and FT43. The prevalence of SCH ranges from 0.4-16.9%, with the majority of those affected being women4. Pregnancy exacerbates the degree of iodine deficiency and therefore increases the risk of SCH5,6. The SCH is a risk factor for endothelial dysfunction and preeclampsia7-9. Elevated TSH is an independent risk factor for endothelial dysfunction and increased intima-media thickness7,10,11, which is a precursor for atherosclerosis and cardiovascular disease10,12. The SCH is associated with the early onset of CVD, increased morbidity and mortality from cardiovascular disease13.
Recently iodine has been shown to have an anti-oxidant capacity 3 times that of vitamin C, one of the most important exogenous antioxidants14. A high oxidant-redox imbalance (oxidative stress) is associated with high circulating levels of oxygen and nitrogen reactive species and endothelial dysfunction and also with increased damage to structural and functional proteins, DNA and lipids, which heralds the onset of cardiovascular diseases and other chronic non-communicable diseases15,16. Therefore, an increased severity of iodine deficiency in pregnancy potentially increases the risk of SCH, TSH and oxidative stress-mediated endothelial dysfunction. It also increases the risk of the onset and severity of preeclampsia and increased intima-media thickness, thereby increasing the risk of future cardiovascular disease.
The World Health Organization recommends a daily dietary intake of at least 250 μg of iodine for pregnant and lactating women17. Since more than 90% of iodine is excreted in urine, spot urine iodine concentration has been recommended as a good measure of (recent) iodine intake2. This case-control study was conducted to ascertain the risk and severity of preeclampsia and carotid intima-media thickness (cIMT) associated with iodine deficiency in pregnancy. The results of this study will help to confirm whether a deficiency of iodine, a known exogenous antioxidant and an essential element for thyroid hormone metabolism, can explain the association between subclinical hypothyroidism, preeclampsia and increased cIMT (a precursor of cardiovascular disease).
MATERIALS AND METHODS
Study design: In this case-control study, retrospective data analysis was carried out on data collected from a cohort of women who attended antenatal care and delivered at the Maternity Unit of Lomo Medical Centre, Kinshasa, Democratic Republic of Congo (DRC) in 2008. The cases were women with preeclampsia. The controls were age-matched pregnant women without preeclampsia at the time of delivery.
Sample size: Two hundred pregnant women who developed preeclampsia while attending antenatal clinic at the Maternity Unit of Lomo Medical Centre, Kinshasa, DRC were consecutively recruited into the study as cases. These were matched for age with 200 controls who delivered at term without having developed preeclampsia. All cases and 150 controls gave informed consent and had complete data were included in the study.
Clinical examination and laboratory methods: Preeclampsia was defined according to the International Society for the Study of hypertension in pregnancy18. Preeclampsia is characterised by the onset of hypertension (>140 mmHg systolic or >90 mmHg diastolic) after 20 weeks’ gestation, with proteinuria (spot urine protein/creatinine >30 mg mmol–1, r>300 mg/day or 2+on dipstick testing), other maternal organ dysfunction, renal insufficiency (creatinine >90 umol L–1, 1.02 mg dL–1), liver involvement (elevated transaminases at least twice upper limit of normal±right upper quadrant or epigastric abdominal pain), neurological complications (such as eclampsia, altered mental status, blindness, stroke, hyperreflexia, severe headaches and persistent visual scotomata), haematological complications (thrombocytopenia-platelet count below 150,000 dL, DIC, haemolysis) and uteroplacental dysfunction (foetal growth restriction, abruptio placentae and intra-uterine foetal death). Severe eclampsia is characterized by >160 mmHg systolic or >110 mmHg diastolic blood pressure with or without systemic organ involvement. Eclampsia is hypertension (>140 mmHg systolic or >90 mmHg diastolic) after 20 weeks’ gestation with convulsions.
T3, T4 and TSH were measured by the enzyme-linked immunosorbent assay kits purchased from DIALAB GmbH IZ-NOE Sued Company, Hondastrasse, Objekt M55, A-2351 wr, Neudorf, Austria. NO was measured using Cayman kits (Cayman Chemical company Ann Arbot, MI). Urinary iodine concentration was measured using the Sandell-Kolthof method. A certified sonographer (BLM) examined the carotid artery B-mode ultrasound imaging using a Biosound Phase (Biosound Inc, Indianopolis, USA), a device equipped with an annular array probe of 7.5 Mhz. Carotid intima-media thickness (cIMT) was measured at the near and far wall interfaces of internal and external carotid arteries.
Ethical considerations: The Ethics Committee of the School of Medicine, University of Kinshasa, granted the approval for this study. Each participant signed a written informed consent form after receiving detailed information on the purpose of the study.
Statistical analysis: Proportions (percentages) of categorical variables were compared using the chi square test, while means of continuous variables, normally distributed, were compared using student’s t-test. The one-way analysis of variance (ANOVA) was used to compare the means of more than two groups. A p-value of <0.05 was considered statistically significant. Data analysis was performed using the Statistical Package for Social Sciences (SPSS) for windows version 23.0 (SPSS Inc) Chicago, Il, USA.
RESULTS
The mean age of the cases and controls was 32.4±6 and 33.5±5.2 years, respectively and p = 0.072 for all cases and controls. Sixty-eight of the 200 cases (68/200) had mild preeclampsia, while 132/200 presented with severe preeclampsia/eclampsia. The mean gestational age at recruitment and sample collection was 37.7 weeks of amenorrhoea (WOA) for controls, 26.9 WOA for mild preeclampsia and 32.5 WOA for severe preeclampsia/ eclampsia.
The mean urinary iodine excretion levels of the women with normal pregnancies (423.9±13.0 μg L–1) was about 1.5 times higher than that of the women with mild and 5 times higher than that of the women with severe preeclampsia/ eclampsia (p<0.0001, Table 1). Although serum T3 and T4 showed a statistically significant increase along the gradient of preeclampsia severity (p<0.0001), they were within normal physiological ranges (Table 1). Serum TSH showed an increase along the gradient of preeclampsia severity, with mild and severe preeclampsia cases having levels above the normal range (Mean±SD of 4.1±0.3 and 6.6±0.2 miU L–1, respectively) but below levels that are diagnostic of overt hypothyroidism (>10 miU L–1). The mean nitric oxide (NO) concentration was 23.0±1.5 μmol L–1 among women with normal pregnancies and significantly reduced along the gradient of severity of preeclampsia (8.7±1.3 and 3.2±0.3 μmol L–1, respectively for cases with mild and severe preeclampsia/eclampsia, p<0.0001) (Table 1). Thus NO was almost 3 times lower among cases of mild preeclampsia and about seven times lower among cases of severe preeclampsia when compared to women with normal pregnancies. Among women with preeclampsia, cIMT showed an average increase of 40% (mean increase of 0.20 mm) above that of women with normal pregnancies. This increase in cIMT was not affected by the level of severity of the preeclampsia.
DISCUSSION
The current study demonstrated that the level of iodine deficiency during pregnancy increases with the risk and severity of preeclampsia. To the researchers’ knowledge, this is the first study in sub-Saharan Africa that has demonstrated the relationship between iodine deficiency, subclinical hypothyroidism, oxidative stress, endothelial dysfunction and cIMT (an early marker of atherosclerosis and cardiovascular disease).
| Table 1: | Analysis of variance of biomarkers |
cIMT: Carotid intima-media thickness, NO: Nitric oxide, UIC: Urinary iodine concentration, TSH: Thyroid-stimulating hormone, T4: Thyroxine, T3: Triiodothyronine, SD: Standard deviation | |
The severity of subclinical hypothyroidism observed in the current study mirrors the pattern of iodine deficiency depicted by the level of mean urinary iodine concentration. This suggests that iodine deficiency is likely to be the underlying cause of SCH in the study population. This is not surprising, given that the study was carried out in DRC, a country with a marked iodine deficiency in the soil, with the potential of less than adequate iodine supplementation for some segments of the population, which may be unmasked by the physiological changes of pregnancy6,19. The risk and severity of preeclampsia follows a similar trend to the severity of subclinical hypothyroidism that is associated with iodine deficiency. This may explain the findings in other studies from iodine-deficient areas that report an increased risk of preeclampsia among women with SCH20,21.
Elevated TSH is a known stimulant of endothelial dysfunction by inhibition of NO synthase7. This is associated with reduced serum NO and a reduction in flow-mediated dilatation and endothelial activation, which are well-known features of preeclampsia22. In the current study, the level of reduction in NO was associated with progressively higher TSH levels, which is associated with the risk of more severe preeclampsia and systemic complications including eclampsia.
Secondly, elevated TSH is associated with increased carotid intima-media thickness (cIMT)8,23. The cIMT is a result of endothelial dysfunction and an early marker of atherosclerosis and future cardiovascular disease24. It is believed that TSH causes increased cIMT by acting on its extrathyroidal receptors (TSHr)25. In the current study, both cases with mild and severe preeclampsia had a 40% increase in cIMT, confirming findings by other researchers that once a woman develops preeclampsia she is at increased risk of future cardiovascular disease. The data seem to suggest that women with mild preeclampsia may be at a similar risk of future cardiovascular disease to those with severe preeclampsia/eclampsia but this requires further investigation in follow-up studies.
Since elevated TSH is associated with increased intima-media thickness25 and elevated serum TSH in the first trimester is associated with an increased risk of preeclampsia9, it is possible that iodine deficiency-mediated SCH with elevated TSH in early pregnancy may predispose to impaired trophoblastic remodeling of the spiral arteries, leading to atherosclerosis of the myometrial spiral arteries and resultant ischemia.
Iodine deficiency in pregnancy may also increase the risk and severity of preeclampsia by tilting the oxidant-redox state in favor of oxidants26. Iodine also reacts with lipids, forming iodolipids, which are less prone to oxidation by oxygen reactive species27. This is of clinical importance in obesity, one of the risk factors of preeclampsia and which is on the rise as a result of nutritional transition28,29. Hence, iodine deficiency will directly diminish the antioxidant capacity within the serum, leading to increasing severity of endothelial dysfunction. This may partially account for the observed increase in the severity of preeclampsia with the diminishing urinary iodine excretion observed in the current study. Iodine deficiency will directly diminish the antioxidant capacity of tissues that normally have high physiological iodine concentrations, including the placenta, thyroid, gastric mucosa and the breast30. Therefore, iodine deficiency is associated with oxidative stress-mediated pathology in these tissues31. In states of iodine deficiency, the oxidative radicals that are normally produced in highly metabolic organs15 like the placenta32 will not be adequately scavenged, leading to further oxidative damage16 and accentuated placenta ischemia; hence the observed increase in the severity of preeclampsia.
The researchers cannot rule out reverse causality in the observed association of thyroid dysfunction with preeclampsia. However, the increasing severity of preeclampsia and SCH with diminishing levels of urinary iodine excretions lends a high possibility of iodine deficiency being the underlying cause, not the effect. The researchers are cognizant of the fact that spot urinary iodine concentration is a measure of recent but not necessarily protracted dietary iodine deficiency. It is unclear whether renal endothelial/glomerular dysfunction that occurs with increasing severity in preeclampsia can reduce urinary iodine excretion in women with an adequate iodine intake.
CONCLUSION
Iodine deficiency in pregnancy is associated with an increased risk and severity of SCH and preeclampsia. The increased risk and severity of preeclampsia may be mediated through the action of TSH on its extra-thyroidal receptors by inhibiting nitric oxide production and leading to increased vascular intima-media thickness, which may be exacerbated by reduced antioxidant capacity secondary to iodine deficiency. Women suffering from preeclampsia may be at increased risk of future cardiovascular disease due to the initiation of the atherosclerotic process during pregnancy, precipitated by SCH and increased intima-media thickness. Therefore, ensuring adequate iodine intake during pregnancy among populations at risk of iodine deficiency is a potential remedy for reducing the risk and severity of preeclampsia and for the prevention of cardiovascular disease.
SIGNIFICANCE STATEMENTS
This study discovers a relationship between iodine deficiency during pregnancy and subclinical hypothyroidism, oxidative stress, endothelial dysfunction and cIMT, all of which are not only associated with preeclampsia but also with future cardiovascular disease. The findings of this study will help to shed light on the potential for maximizing iodine nutrition in pregnancy and the reproductive years, especially in areas with endemic iodine deficiency, as a measure for the prevention of preeclampsia and cardiovascular disease among women and their progeny.