First-Trimester Reference Intervals for Thyroid Function Testing among Women Screened at a Tertiary Care Hospital in India

Introduction

Thyroid hormones (TH) are crucial for embryogenesis and fetal development. During the first half of the pregnancy, they are sourced entirely from the TH circulating in the maternal blood.^[1] Thyroid dysfunction during pregnancy increases the risk of complications such as pregnancy loss, premature delivery, gestational diabetes, gestational hypertension, low newborn birth weight, and impaired neuropsychological development.^[2]

During pregnancy, the increased demand for TH for fetal development, increased concentration of thyroxine-binding globulin due to increase in estrogen levels, and inactivation of TH by placental iodothyronine deiodinase type 3 necessitate the increased production of maternal TH.^[1] The pregnancy hormone, β-human chorionic gonadotropin, is a weak thyroid-stimulating hormone (TSH) receptor agonist. It also stimulates the thyroid gland to compensate for the increased degradation of TH.^[1] TSH level decreases and the free thyroxine (fT4) level in the maternal blood increases. Hence, there is a need for determining gestation-specific reference intervals (GRIs) for normal thyroid function. This may help to identify women who can potentially benefit from timely treatment.

Studies done at a few centers cannot be extrapolated to all centers globally due to differences in method of assay as well as population-specific factors such as ethnicity, body mass index (BMI), and iodine intake. The reference intervals for thyroid function tests in the same woman also vary from one trimester to another. The International Federation of Clinical Chemistry (IFCC), the International Committee for Standardization in Hematology, the Endocrine Society, the European Thyroid Association, and the American Thyroid Association (ATA) advise every center to determine their own population-based trimester-specific GRIs.^[3-6]

Initial recommendations by ATA and the European Endocrinology Society had set the TSH reference limits of 0.1 to 2.5 mIU/L for the first trimester.^[7] The ATA guidelines revised in 2017 recommend that pregnancy-specific and laboratory-specific reference ranges for TSH and fT4 should be calculated. If that is not possible, a reference range can be used from the literature that is derived from a population with similar ethnicity, BMI, iodine status, etc.^[8] If both are not possible, ATA recommends the first-trimester upper cutoff limit 0.5 mIU/L less than the pre-pregnancy TSH value. If the pre-pregnancy value is unavailable, the upper limit is taken to be 4.0 mIU/L.^[7,8]

The definition of a thyroid disorder tends to be dependent on the TH level because of which the top 2.5% and the bottom 2.5% of people are believed to have thyroid dysfunction. This can be misleading and lead to unnecessary treatment for many people. Expressing TH levels as multiple of median (MoM) values can allow for interlaboratory assay variations, gestational age, and other factors that may affect the marker levels.^[9] To unitize laboratory reports and aid in uniform interpretation of results, MoM values may be incorporated in reporting. MoM measures how far the result deviates from the median. This value is the ratio of the patient's result and the median result appropriate for the gestational period.

Maturation of the fetal thyroid function involves proper functional development of the hypothalamus, pituitary gland, and the thyroid gland. This process is more or less complete by the 12th to 14th week of gestation.^[10] After the 14th week of gestation, there may be irreversible effect on the fetal brain development due to lack of adequate TH in the maternal blood.

Some studies show that the median TSH in early first trimester (4–6 weeks) is significantly higher (p < 0.001) than that in the late first trimester (7–12 weeks).^[11] The Polish Society of Endocrinology recommends universal thyroid screening of all women either preconception or at their first antenatal visit (4–8 weeks).^[12] However, the current ATA guidelines cite unclear evidence in support of universal thyroid screening. They strongly recommend clinical evaluation of all patients, who seek pregnancy or are newly pregnant, for identifying risk factors of thyroid disorders.^[13] Only those with risk factors for thyroid disorders are then tested for thyroid function.

In this study, we determined the first-trimester GRIs of TH for pregnant women in Chhattisgarh state in India. We studied if the GRIs changed depending on whether the TH levels were determined in the early weeks (4–7th week) or the late weeks (7–14th week) of the first trimester. We also calculated the MoM values of TSH and fT4.

Materials and Methods

Results

Discussion

In our study, in the first trimester, the GRI of serum TSH was 0.25 to 4.97 mIU/L. The upper limit was higher than that mentioned in ATA guidelines as well as obtained from several other studies (►Table 5). This may be due to the difference in study design, sample size used, ethnicity, region, and/or assay method.

Low maternal TSH that is still detectable is unlikely to be of clinical significance. However, since the normal upper limit of maternal TSH in our study population is higher than that set by ATA guidelines, the use of the GRI of ATA for pregnant women attending our institute could lead to misdiagnosis and mistreatment in many women. Overtreatment with TH can hinder proper fetal brain development and increase the risk of child mental health disorder, ADHD, and autism spectrum disorder.^[18] Therefore, we needed to set our own institutional GRI.

The definition of thyroid dysfunction is also dependent on the fT4 concentration. In our study, serum fT4 reference range during the first trimester was from 10.2 to 18.8 pmol/L. Rajput et al^[19] reported first trimester serum fT4 range as 0.88 to 1.78 ng/mL that is equivalent 1,132.560 to 2,290.860 pmol/L. These values are almost 100 times more than ours and most other studies (►Table 5). We wonder if there was an error in the use of units by Rajput et al.^[19] They report their fT4 values in ng/mL and not in ng/dL as done by researchers of other Indian studies.^[20-22]

When fT4 is within normal limits but TSH level is persistently above the normal limit, a biochemical diagnosis of subclinical hypothyroidism can be confirmed. TSH levels within normal range of laboratory do not always mean the individual has normal thyroid function.^[7] The TSH value may be outside the individual reference range. However, it is impractical to take several samples and measure individual reference range for each patient. Laboratory-specific reference range is preferred. It is not difficult to calculate laboratory-specific reference range. Every hospital where perinatal care is provided can collaborate with its clinical biochemistry department and conduct a study to determine their own laboratory-specific GRIs for TSH and fT4.

Andersen et al^[23] and Laurberg et al^[1] report that the use of uniform reference range for TSH and fT4 in early pregnancy may be too simple and can lead to frequent misclassification and incorrect use of therapy. They suggest the use of a reference range stratified by the week of pregnancy because of the dynamic physiological changes seen over weeks. We observed no significant difference in the TSH and fT4 reference ranges among women in early or women in late first trimester. Khalil et al^[24] had also found no difference in the 4 to 6 weeks and 7 to 12 weeks gestation groups.

TSH ranges can vary slightly depending on the method of analysis used. This variability can be reduced by using MoM values.^[13] MoM is often used for screening test results. Computer software is available that can convert marker levels of Down syndrome or neural tube defects to MoM values. Similar software can be used to calculate MoM values for TSH and fT4 levels. Data from approximately 100 patients would be sufficient.^[9] We have used data from 341 patients to determine the MoM values.

An individual value is divided by the population median and the resulting value is not influenced by the method of assay used. Reporting the MoM value along with the centile, it corresponds to, does not put a woman into the abnormal category that can happen when only the reference range is used.^[9] MoM values also take into account certain factors such as gestational age and maternal weight that can have a relationship with TSH and fT4 values.^[9]

Considering obesity is increasing in many populations, it may be an important finding that maternal weight correlated with TSH MoM values (p = 0.027, r = 0.120). Bestwick et al^[9] had found that for every 10 kg increase in weight, there was 0.025 MoM increase in TSH MoM values and 0.009 MoM decrease in fT4 MoM values. In contrast, we found no significant correlation between maternal weight and fT4 MoM values (p = 0.749, r = − 0.017).

The 1st to 99th percentiles of TSH and fT4 MoM values in our study were 0.03 to 4.09 and 0.66 to 1.39, respectively. In a study by Kianpour et al,^[25] among pregnant Iranian women with gestational age ≤ 14 weeks, the corresponding values were 0.06 to 4.62 and 9.00 to 18.02, respectively. In the study by Bestwick et al,^[9] among pregnant women (gestational age less than 16 weeks) from United Kingdom, the 1st to 99th percentiles of TSH and fT4 MoM values were 0.02 to 4.13 and 0.75 to 1.38, respectively. Among those from Italy, corresponding values were 0.01 to 3.93 and 0.76 to 1.46, respectively. Both our TSH and fT4 MoM results match those from the two populations studied by Bestwick et al,^[9] whereas only our TSH MoM values match those from the study by Kianpour et al.^[25] The first-trimester–specific reference interval for serum fT4 is similar in both our study and that by Kianpour et al, but the fT4 MoM values are grossly different. A careful look at the fT4 MoM values published by them in a 2019 issue of Hormone and Metabolic Research suggests that there might have been some error as the numerical values of MoM are exactly the same as the individual centile values for fT4.^[25]

To the best of our knowledge, ours is the largest Indian study of women in their first trimester that determines the laboratory-specific GRIs for TSH and fT4. It is also probably the first one in South East Asia to determine the MoM values for TSH and fT4 during the first trimester. Another strength of our study was the measurement of anti-TPO, urine iodide, and detection of multiple gestations through ultrasound scanning. It helped in a clearer definition of the normal population for reference ranges. We could also have attempted to determine the GRIs for the second and third trimester using this sample population. Though our sample size of 341 pregnant women in their first trimester is perhaps the largest in India, a larger sample size could have provided more accurate reference intervals.

To obviate the slightly different, although overlapping, reference ranges of various commercially available assays, we have used the MoMs. This may allow other laboratories to use our specific MoM thresholds to their centers once they have established their own first trimester-specific medians. Further research is needed to validate the extrapolation of our specific MoM values to other centers.

Conclusion

In this study, GRIs of serum TSH and fT4 were determined for singleton pregnant Indian women with anti-TPO less than 60 kIU/L and adequate level of iodine. The GRIs determined in our study are different from that of ATA and the GRIs determined by other studies (►Table 5). Our laboratory-specific reference ranges may be used for the pregnant women in Chhattisgarh. Once first-trimester median values are obtained by a laboratory, it will also be able to use our MoM values as cutoffs to obviate differences in the assay methods. This can aid accurate detection of thyroid disorders during pregnancy in a specific population.