A Study Estimating the Serum TSH Levels in First Trimester Pregnant Women

Introduction

During pregnancy, several physiological changes occur that affect the thyroid. These include heightened renal iodine clearance, a rise in serum thyroxine-binding globulin (TBG) due to oestrogen, stimulation of the thyroid by human chorionic gonadotropin (hCG), and increased production of thyroid hormones.¹

Fluctuations in hormones are widespread during pregnancy. These changes encompass not just oestrogen and progesterone levels, but also other crucial hormones such as thyroid hormones, vital for typical growth and the development of skeletal system.

Thyroid hormones (TH) are crucial for embryogenesis and foetal development. During the first half of the pregnancy, they are sourced entirely from the TH circulating in the maternal blood.² Hypothyroidism is a prevalent thyroid disorder during pregnancy, affecting approximately 3-5% of pregnant women.³ So it is very important that we maintain TSH values within normal gestation-specific reference range.⁴

Trimester-specific reference intervals for TSH are recommended to assess thyroid function during pregnancy due to changes in thyroid physiology. Maternal thyroid dysfunction during pregnancy increases the risk of pregnancy-related complications for both mother and child. These include miscarriage, hypertension, placental abruption, intra-uterine growth restriction, premature birth, low birth weight and impaired neurodevelopment. The risk of complications is most pronounced for overt maternal hypothyroidism and TSH has an important role in baby’s brain development during pregnancy.⁵ So screening for thyroid disorder in early pregnancy can help to diagnose the cases and prevent adverse prenatal outcomes. Routine screening of TSH should be a part of screening protocol so that all thyroid disorders are screened and treated at the earliest.⁶ Therefore, we aimed to study the TSH levels and determine the prevalence of various thyroid derangements in early pregnancy according to the current reference ranges available.

Materials and Methods

The research focused on pregnant women attending the obstetric outpatient department at St. John’s Medical College & Hospital, Bangalore throughout January to December, 2021. A retrospective study was conducted by gathering data from the clinical Biochemistry Laboratory during the same period. Healthy pregnant women aged 20 to 35 years were included in the study.

The women whose TSH levels were unavailable from the data, alongside a history of systemic illness, endocrine disorders, and complications in previous pregnancies were excluded from the study. TSH information was extracted from the laboratory information system, complemented by detailed medical and obstetrics records. Our analysis specifically isolated data pertaining to TSH values in the first trimester without complications, resulting in a cohort of 228 pregnant women aged between 20 and 35 years. Serum TSH levels were measured using the ABBOTT ARCHITECT CI8200 employing Chemiluminometric Immunoassay. Subsequently, the collected data underwent thorough analysis and comparison to derive descriptive statistics.

Statistical analysis

Data was analysed using statistical software packages for social science (SPSS). All study variables were described by using descriptive statistical method like frequency, percentage, mean and standard deviation.

Results

A total of 228 expectant mothers in their first trimester, meeting the inclusion criteria, were part of this study. The participants' maternal age ranged from 20 to 35 years. Among these women, 82 were in the age group of 20-25 years (35.9%), 101 were in the age group of 25-30 years (44.2%), and 45 were in the age group of 30-35 years (19.7%).

Regarding TSH levels, among the 228 pregnant women, 12 displayed low TSH values (<0.35 µIU/mL), 11 had higher values (>4.94 µIU/mL), and 205 exhibited normal TSH levels during their first trimester.

The prevalence rates estimated in this study were 5.3% for hyperthyroidism and 4.8% for hypothyroidism, as depicted in Table 2.

Additionally, Table 3 shows the statistical analysis of TSH levels in the first trimester. This study's mean, median, and percentiles (2.5th and 97.5th) for TSH levels were observed as 1.94, 1.51, 0.23, and 5.86 µIU/ mL. The TSH values obtained throughout this study averaged at 1.94±1.50 µIU/mL

Discussion

The research aimed to explore the correlation between TSH levels during the first trimester among healthy, low risk pregnant women without prior thyroid condition. One of the early Indian reports on reference ranges for TSH states similar results as in our study.⁴

Our findings revealed that among the 228 pregnant women included, 205 showed normal TSH levels, 12 pregnant women suffered from hyperthyroidism and 11 of them had hypothyroidism. Thus, the total number of pregnancies affected by thyroid dysfunction accounted for only 10.08% of all pregnancies.

Another study done by Katia Vella et al. reported an incidence of 1.3% thyroid dysfunction⁷

Rana Turkal et al.,¹ carried out a study where they observed that implementary trimester base RIs instead of using fixed cut off values could reduce the thyroid dysfunction prevalence from 19% to 16.5% in the first trimester improving the clinical assessment of 13% of women who are considered to have abnormal thyroid function.¹

The findings of a recent study conducted by Małgorzata Gietka Czernel and Piotr Glinicki from the Department of Endocrinology at the Centre of Postgraduate Medical Education in Warsaw, Poland, underscore the importance of establishing trimester-specific reference ranges for thyroid stimulating hormone (TSH) and free thyroid hormones during pregnancy.⁸

Various authors suggested that the time of diagnosis and management of thyroid dysfunction could be a contributing factor to the pregnancy outcome as some believe that early diagnosis and management improves the outcome of pregnancy for both the mother and the baby.⁹

Samuel Chigbo Obiegbusi et al. found a significant association between maternal age and abortion among pregnant women diagnosed with subclinical hypothyroidism and who developed intrauterine growth restriction after being diagnosed with hypothyroidism in the second trimester.⁹ Although the correlations between TSH levels and spontaneous miscarriage have been reported worldwide, the conclusions are controversial.¹⁰

Anupama Dave et al. in their study indicated a significant link between risk factors and hypothyroidism, suggesting the necessity of high-risk screening during early pregnancy. However, focusing solely on high-risk populations would overlook 4.6% of cases that could have been identified and treated earlier. Therefore, the study underscores the importance of high-risk screening in early pregnancy while advocating for the integration of universal screening into screening protocols. This approach aims to identify and address all thyroid disorders promptly, emphasizing the importance of not missing even the smallest percentage of thyroid disorders during early pregnancy. Consequently, adhering to the Indian Thyroid Society Guidelines is crucial, as they recommend screening all pregnant women for TSH levels during their first antenatal visit, ideally during pre-pregnancy evaluation or immediately after confirming pregnancy.⁶

The limitation of this present study is that the period of study was of short duration and the prevalence determined encompassed the thyroid dysfunction altogether. Additionally, insufficient sample size stands as another limitation. The retrospective nature introduces uncertainties regarding treatment disparities among women. Recognizing these limitations underscores the need for cautious interpretation and encourages future research to address these constraints.

Conclusion

The observed connection between aberrant TSH serum levels during the first trimester and adverse pregnancy outcomes underscores the necessity for proactive screening. Identifying pregnant women with elevated or reduced TSH levels early on holds significant potential for preventing complications. The incorporation of such screening protocols can yield a range of benefits, including timely interventions and enhanced management strategies to mitigate adverse outcomes. Our research contributes to the growing body of evidence supporting TSH screening during early pregnancy, emphasizing its pivotal role in safeguarding maternal and foetal health.

Conflict of Interest

Nil

Acknowledgment

We are highly indebted to Dr. Geraldine Menezes, Professor in the Department of Biochemistry for giving the moral support to complete the project.