Thyroid functions and insulin resistance in pregnant Sudanese women

Abstract

Background

The thyroid function test (free triiodothyronine [FT3], free thyroxine [FT4], and thyroid-stimulating hormone [TSH]) is one of the key determinant of glucose homeostasis by regulating the balance of insulin. Thyroid dysfunction alters glucose metabolism, leading to insulin resistance (IR). This study aimed to assess the association between thyroid function and IR in pregnant Sudanese women.

Method

A cross-sectional study was conducted in Saad Abuelela Hospital, Khartoum-Sudan, from January to April 2021. Obstetric/sociodemographic characteristics were gathered through questionnaires. Serum TSH, FT3, FT4, fasting plasma glucose (FPG), and fasting insulin levels were measured and evaluated, and IR was estimated using the homeostatic model assessment for insulin resistance (HOMA-IR) equation.

Results

In total, the study included 127 pregnant women with a median age of 27.0 years (interquartile range [IQR] 23.0‒31.2) and a median gestational (IQR) age of 25.0 (IQR 25.0‒27.0) weeks. The medians (IQRs) of the TSH, FT3, and FT4 were 1.600 (1.162‒2.092) IU/ml, 2.020(1.772‒2.240) nmol/l, and 10.70 (9.60‒11.90) pmol/l, respectively. The median (IQR) of the FPG and fasting blood insulin level was [69.0 (62.00‒78.00) mg/dl] and [5.68(2.99‒11.66) IU/ml], respectively. The median (IQR) of the HOMA-IR level was 0.9407 (0.4356‒2.1410). There was a positive correlation between HOMA -IR and FT3 levels (r = 0.375; P < 0.001) and a negative correlation with FT4 levels (r= -0.312; P < 0.001). Also, a significant positive correlation was found between fasting insulin levels and FT3 levels (r = 0.438; P < 0.001) and a negative correlation with FT4 levels (r= -0.305; P < 0.001).

Conclusions

This study indicated that FT3 has positive correlation with HOMA-IR, while FT4 has negative correlation among healthy pregnant women without a history of thyroid dysfunction. This may indicate screening of euthyroid pregnant women for thyroid dysfunction and IR. Further studies are needed.

Background

Pregnancy is a physiological process accompanied by multiple physiological and metabolic changes [1]. Normal pregnancy requires an escalating need for nutrients to fulfill the growing foetus energy requirements. Therefore, insulin hormone levels start to increase, as well as blood glucose levels, which can lead to a state of insulin resistance (IR) [2]. The exact cause of maternal IR remains obscure; however, many factors are associated with the development of IR, such as a sedentary lifestyle, obesity, genetics, and epigenetics [2]. Another physiological change that occurs during pregnancy is altered iodine metabolism. This alteration is due to increased renal loss, secondary to the general increase in glomerular filtration rate during pregnancy [3]. During the first trimester, the foetus depends solely on the maternal thyroid gland to secure sufficient thyroid hormone [2]. The thyroid hormones triiodothyronine (T3) and thyroxine (T4) fine-tune the balance of glucose homeostasis by supporting and contradicting insulin hormone actions. Experimental studies have revealed that T3 and T4 upregulate the gene expression of hepatic enzymes essential for glycogenolysis and gluconeogenesis pathways and downregulate the expression of enzymes needed for glycogen synthesis [4]. Hence, they antagonize the hypoglycaemic action of the insulin hormone. On the other hand, thyroid hormones upregulate the expression of glucose transporter-4 (GLUT4), which is expressed in peripheral tissues and facilitates glucose entry into the muscular and adipose tissues, thereby enhancing the hypoglycaemic action of insulin [5]. Therefore, any disturbance in thyroid hormone levels can dysregulate and alter glucose metabolism. As a result, thyroid dysfunction may be considered an additional source of IR [6,7,8,9]. Clinically, administering thyroid hormone in a dose sufficient to suppress the TSH improves insulin sensitivity and glucose metabolism in a patient with an insulin resistance state [10]. This has also been confirmed at the animal model level [11]. This may provide a rationale to hypothesize that pregnant women at risk of IR may benefit from supplemental doses of thyroid hormone during pregnancy. Maternal thyroid dysfunction can lead to adverse pregnancy outcomes, namely gestational diabetes mellitus and preeclampsia [12,13,14]. These complications have been attributed to IR [15,16,17]. Moreover, thyroid dysfunction may also precipitate adverse foetal outcomes such as miscarriage, neonatal death, intrauterine growth restriction and foetal neuropsychological development [18]. Given the crucial physiological and pathological roles of thyroid hormones and their interplay with metabolic changes during pregnancy, assessing the thyroid hormone function may serve as a useful indicator IR in pregnant women. Nonetheless, many pregnant women with IR have a normal course of pregnancy without any maternal-adverse outcomes [19]. Perhaps disturbances in IR indices preceded tissue damage and clinical complications [20].

Since normal thyroid hormone levels in a specific community is affected by genetic make-up, iodine intake, seasonal variations, and other environmental conditions, reference range for thyroid hormones are established based on specific regions and communities [21,22,23]. A limited number of studies have investigated the association of IR indices with thyroid function during pregnancy [13, 24], and no African studies have been conducted so far. Substantial studies are needed to establish an association between thyroid function status and IR in pregnancy. Such associations help clinicians identify and provide better care for pregnant women who are at increased risk of developing known IR-related complications such as gestational diabetes and preeclampsia. Moreover, we believe that the findings of this study will add to the body of African literature about the association of thyroid hormone function with IR in euthyroid pregnant women. Therefore, we aimed in this study to investigate the association between indices of IR and components of thyroid function tests (free T4 [FT4], free T3 [FT3], and thyroid-stimulating hormone [TSH]) among healthy pregnant women in early pregnancy.

Methods

Study design and study settings

This was a hospital-based cross-sectional study conducted between January–April 2021 at Saad Abuelela Maternity Hospital. Saad Abuelela Maternity Hospital is a tertiary semi-private hospital directed by the Faculty of Medicine, University of Khartoum. It is located in Khartoum, the capital of Sudan. The pregnant women were instructed to come fasting for their next scheduled visit to perform an oral glucose tolerance test (OGTT) and screen for gestational diabetes. Pregnant women planning to have an OGTT during their antenatal care visits (24–28 weeks of gestation) were approached to participate in the study. The objectives of the study were briefed to the participants, and signed consent was obtained. Sudanese pregnant women with singleton pregnancies who were willing to participate in this study were enrolled. Women with a history of thyroid disorder and/or current use of thyroid medication, with diabetes mellitus, hypertension, or any other chronic disease or acute illness, were excluded from the study. Women who refused consent to participate and women with multiple pregnancies were also excluded from the study. Obstetric/sociodemographic characteristics and medical histories were gathered from the participants using questionnaires. The duration of gestation was calculated based on the dates of their last menstruation and confirmed by ultrasonography. Body mass index (BMI) was computed based on height and weight, which were measured using the standard method.

Methods of sampling and laboratory testing

Venous blood from the antecubital vein was collected from each pregnant woman into plain tubes, allowed to clot, centrifuged to separate the serum, and stored at -20 °C until analyzed for measurements of serum thyroid profile (FT3, FT4, and TSH), OGTT and serum fasting insulin.

Maternal serum thyroid profile (FT3, FT4, and TSH) and fasting insulin levels were measured using the immunoassay analyzer AIA 360 (Tosoh Bioscience, San Francisco, CA, USA), following the manufacturer’s instructions. The assay is a competitive chemiluminescence enzyme immunoassay performed in an assay test cell. The cell is coated with a hormone-specific monoclonal antibody that binds with patient serum hormones (FT3, FT4, TSH, and insulin). A specific volume of an enzyme-labeled hormone (T3, T4, TSH, and insulin) competes with the bound or free hormone-specific monoclonal antibody. After incubation, the cell is washed to remove the unbound enzyme-labeled hormone and then incubated with a specific chemiluminescent substrate named DIFURAT. A standard curve was constructed using six calibrators with escalating concentrations for each hormone, and the concentration of the test samples was quantified. The reported thyroid hormone reference range for Sudanese pregnant women were as follows: TSH (0.079–2.177 IU/ml), FT3 (3.843–6.562 nmol/l), and FT4 (13.02–31.48 pmol/l), respectively [25]. Using the hexokinase method, the FPG levels were estimated by Hitachi 902 auto-analyzer (Roche Diagnostics GmbH, Mannheim, Germany).

Assessment of insulin resistance by homeostatic model assessment for insulin resistance (HOMA-IR)

Homeostatic model assessment (HOMA-IR) is a method for assessing β-cell function and peripheral insulin resistance from fasting insulin and fasting glucose concentrations. Matthews et al. first used HOMA-IR to assess IR [26]. It is a mathematical model for assessing insulin resistance.

The HOMA-IR equation is as follows:

Fasting plasma glucose [mg/dL] × fasting insulin [µU/mL] / 405.

The physiological value of the index was 1.0. Higher values are indicative of insulin resistance. Low HOMA-IR indicates good insulin sensitivity. A small amount of insulin is sufficient to maintain glucose homeostasis, while a higher HOMA-IR denotes more IR. The healthy IR range is 0.5–1.4; < 1.0 indicates optimal insulin sensitivity; > 1.9 indicates early IR; and > 2.9 indicates significant IR [27].

Sample size calculation

A sample size of 127 women was calculated depending on the significant minimum level (0.25) for correlations between the serum insulin, glucose levels, and thyroid function hormones (TSH, FT3, and FT4) variables [28]. This sample had 80% power and a difference of 5% at α = 0.05.

Statistical analysis

All statistical analyses were performed using the Statistical Package for the Social Science software for Windows (SPSS Inc., Chicago, Illinois, USA, version 22.0). All Continuous variables were checked for normality using the Shapiro–Wilk test. Continuous data were expressed as median (interquartile range), as they were not normally distributed. Spearman’s correlation was used to investigate the correlation between HOMA-IR score, insulin levels, plasma glucose, and thyroid profile. A Spearman’s correlation coefficient and coefficient of determination were reported. A P-value of < 0.05 was considered significant.

Results

Characteristics of the participating women

In total, 127 apparently healthy pregnant women with insulin resistance and a median (IQR) age of 27.0 (23.0‒31.2) years participated in this study. The median (IQR) of parity, gestational age, and BMI was 1(1‒3), 25.0 (IQR 25.0‒27.0) weeks, and 26.6 (24.0‒29.7) kg/m², respectively. The median (IQR) of the TSH, FT3, and FT4 was 1.600(1.162‒2.092) IU/ml, 2.020(1.772‒2.240) nmol/l, and 10.70 (9.60‒11.90) pmol/l, respectively. The median (IQR) of FPG was 69.0(62.0‒78.0) mg/dl, and the median (IQR) of fasting blood insulin level was 5.68(2.99‒11.66) IU/ml. The HOMA-IR was calculated from fasting plasma glucose and fasting blood insulin values. The median (IQR) of the HOMA-IR level was 0.940 (0.435‒2.141; see Table 1).

Table 1 General characteristics of Sudanese women in early pregnancy (n = 127)

Correlation analysis

Pearson’s correlation analyses exhibited a significant, strong direct correlation between FT3 and HOMA-IR (r = 0.418; P < 0.001) and between FT3 and fasting insulin level (r = 0.438; p < 0.001). A significant inverse correlation was observed between FT4 levels and HOMA-IR (r = -0.284; p < 0.001) and between FT4 and fasting insulin levels (r = -0.305; p < 0.001; Table 2).

Table 2 Correlation analysis between fasting insulin level, HOMA-IR, and thyroid profile (n = 127)

Discussion

The main finding of this study was that FT4 had a negative correlation with HOMA-IR. This finding is in agreement with the findings of Bassols et al., who investigated normal pregnant women with euthyroid status [24]. This is also in concordance with Knight et al.’s findings, which included a group of apparently healthy Caucasian pregnant women with singleton and without diabetes mellitus [13]. They reported a negative correlation between FT4 and HOMA-IR. This correlation indicates that low FT4 levels decrease tissue sensitivity to insulin hormones and hence precipitate/exaggerate insulin resistance. Among their metabolic functions, thyroid hormones greatly affect the regulation of glucose homeostasis and contribute to the regulation of pancreatic function and secretion [29, 30]. Thyroid hormones are major metabolic hormones that ameliorate IR and reduce hyperglycemia and hyperinsulinemia in a type 2 diabetes mouse model [11]. At the cellular level, thyroid hormones adjust the gene expression of insulin-sensitive GLUT-4 in muscles and adipose tissues. They also increase the gene expression of hepatic enzymes essential for gluconeogenesis and glycogenolysis pathways, including glucose 6-phosphatase, phosphoenolpyruvate carboxykinase (PEPCK), and pyruvate carboxylase [31]. Moreover, at very high doses, thyroid hormones can improve blood glucose levels and enhance insulin sensitivity in a diabetic subject harbouring insulin receptor gene mutation and with an extreme insulin resistance level [10]. Taken together, this basic and experimental evidence explains – at least partially – the impact of thyroid hormones on glucose metabolism and insulin resistance.

Another important finding in this study was the negative correlation between FT4 and insulin levels and the positive correlation between FT3 and insulin levels. Our finding is in line with Bassols coworkers findings in euthyroid pregnant women [24], Lacobellis et al., findings in euthyroid non-pregnant obese women, and with Kocatürk et al., in a group of insulin resistance type 2 patients [32]. This can be partially explained by the fact that, during pregnancy, the placenta ensures maximum transportation of total T4 and T3 toward foetal circulation [33]. Moreover, the activity of the 2-iodothyronine deiodinase enzyme, which converts T4 to T3, is markedly induced during pregnancy, especially when levels of T4 are low [34]. Furthermore, the placenta expresses type 2-iodothyronine deiodinase, which ensures further conversion of T4 to T3, hence further reduction in T4 levels and elevation of T3 levels [33]. These arguments collectively result in a massive reduction in maternal T4 hormone and increased T3 levels. The exact mechanism underlying the positive correlation between FT3 and insulin levels, as well as the negative correlation between FT4 and insulin levels, in relation to insulin resistance remains unclear. However, in euthyroid subjects FT3 has been reported to be associated positively with insulin hormone levels and fasting blood glucose levels, which implicate its involvement in the impaired fasting blood glucose and hyperinsulinemia—key features of insulin resistance [35, 36]. Serum FT3 has been found to be consistently higher in patient with GDM compared to normal control, yet within the euthyroid range [37]. In euthyroid pregnant women with insulin resistance, Bassols and colleagues observed an inverse correlation between FT4 levels and glycated hemoglobin suggesting that reduced FT4 may decrease peripheral tissue sensitivity to insulin. This reduction in sensitivity could lead to diminish glucose utilization, contributing to hyperglycemia and hyperinsulinemia [24]. FT4 also has been linked to metabolic dysfunction in euthyroid pregnant women, including increased HOMA-IR and a deteriorated lipid profile [24]. Collectively, these premises may justify the association of a positive correlation between FT3 and insulin levels and the inverse correlation between FT4 and insulin levels as possible factors contributing to insulin resistance.

Both the thyroid hormone function test and insulin hormone index measurements were not routine tests during the antenatal care visit. However, our study concluded that an euthyroid state may not indicate an entirely normal pregnancy. Care providers should consider including these tests in routine investigations of pregnant women.

This study has some limitations that should be addressed for a better interpretation of our findings. First, the design is a cross-sectional study; therefore, the outcomes of the pregnancies remain unknown. Second, we recruited pregnant women during the second trimester of their pregnancy, and due to a lack of previous medical records, we reported pregnancy BMI instead of pre-pregnancy BMI. Third, although we measured the thyroid hormone using standard methods, the reference range varied between populations based on genetic, sociodemographic, and gestation-specific factors, among others.

Conclusions

To conclude, this study documented a positive correlation between FT3 and IR and a negative correlation between FT4 and IR in a group of pregnant Sudanese women with euthyroid hormone profiles. These findings highlight the importance of screening for thyroid dysfunction early in pregnancy, which could help identify cases of IR and thyroid dysfunction at an early stage. Such early detection allows gynaecologist and endocrinologist to prioritize high-risk pregnant women, providing them with more frequent follow-up visits and early possible intervention to prevent IR-related complications. Additionally, the detection of increased IR indices may warrant further investigation into thyroid function in these pregnant women. Therefore, more studies are needed with longitudinal design and recruiting women before conception and follow them up to delivery to establish the association of IR with thyroid dysfunction throughout the course of pregnancy.