Anti-thyroid antibodies and underlying generalized immunologic aberrations in patients with reproductive failures

The link between autoimmune thyroid diseases and reproductive failures, including implantation failure and pregnancy loss, has been attracted a great deal of attention in the last two decades. In this regard, a considerable progress has been achieved in understanding the etiopathogenesis of the adverse pregnancy consequences related to the presence of anti-thyroid antibodies, however, the exact action mechanisms of these antibodies are not fully comprehended. Thyroid peroxidase antibodies (TPOAbs), thyroglobulin antibodies (TgAbs) and TSH receptor antibodies (TRAbs) are the anti-thyroid antibodies which are present in autoimmune thyroid disorder (AITD) patients, such as Hashimoto's thyroiditis. In this condition, the thyroid hormones production, which are essential for normal implantation and pregnancy, are disrupted, and compromise the embryo or fetus development. In addition, a hypothesis suggests that there is underlying generalized immune abnormalities behind the presence of these antibodies. On the other hand, similar immunologic aberrations have been observed in thyroid autoimmunity and reproductive complications, which are postulated to be the proper answer for the scientists who seek for the pathophysiology behind the presence of these antibodies. Elevated inflammatory responses and decreased immunoregulatory actions, seem to be the main interfering pathologic factors in maternal tolerance toward fetus. In addition, cross reactivity of these antibodies with antigenic determinants of egg, embryo and placenta is another suggested mechanism, causing implantation and pregnancy complications. The ability of anti-thyroid antibodies in passing through the placental barrier and affecting the fetal thyroid gland, makes them more threatening for maintenance of a pregnancy.

Establishment of trimester-specific reference intervals for thyroid stimulating hormone and free thyroxine during pregnancy in southwest China by indirect method

OBJECTIVE

A series of physiological changes in thyroid function occur during pregnancy and differ from those non-pregnant women. This study aimed to establish the pregnancy-specific reference intervals of TSH and FT4 using an indirect method based on the healthy pregnant women from southwest China population.

METHODS

Thyroid function test results which available on the Laboratory Information System (LIS) were collected from the pregnancies who visited the Obstetric Clinic or the Department of Gynecology between 1 January 2015, and 30 December 2020. We grouped the data by trimesters to establish the reference intervals (RIs) based on the clinical consensus of different levels of TSH and FT4 at different weeks of gestation. All arrangements were referenced to the document CLSI EP28-A3C.

RESULTS

A total of 33,040 thyroid function test results of pregnant women, aged 31 (28,33) years were statistical analyzed. Estimated RIs for TSH and FT4 in the first, second and third trimesters corresponding to the 2.5^th and 97.5^th percentiles in TPOAb negative were 0.02-5.23, 0.03-5.24, 0.37-5.68 mIU/L, 11.66-20.69, 10.1-18.59, 9.85-16.86pmol/L, respectively.

CONCLUSION

This study provides trimester-specific RIs for TSH and FT4 among healthy pregnant women in southwest China which guides clinicians to diagnosis and screen for thyroid disorders in this region.

Method-dependent variation in TSH and FT4 reference intervals in pregnancy: A systematic review

BACKGROUND

Gestational TSH and FT4 reference intervals may differ according to assay method, but the extent of variation is unclear and has not been systematically evaluated. We conducted a systematic review of published studies on TSH and FT4 reference intervals in pregnancy. Our aim was to quantify method-related differences in gestation reference intervals, across four commonly used assay methods, Abbott, Beckman, Roche and Siemens.

METHODS

We searched the literature for relevant studies, published between January 2000 and December 2020, in healthy pregnant women without thyroid antibodies or disease. For each study, we extracted trimester-specific reference intervals (2.5-97.5 percentiles) for TSH and FT4 as well as the manufacturer-provided reference interval for the corresponding non-pregnant population.

RESULTS

TSH reference intervals showed a wide range of study-to-study differences with upper limits ranging from 2.33 to 8.30 mU/L. FT4 lower limits ranged from 4.40 to 13.93 pmol/L, with consistently lower reference intervals observed with the Beckman method. Differences between non-pregnant and first trimester reference intervals were highly variable, and for most studies, the TSH upper limit in the first trimester could not be predicted or extrapolated from non-pregnant values.

CONCLUSIONS

Our study confirms significant intra- and intermethod disparities in gestational thyroid hormone reference intervals. The relationship between pregnant and non-pregnant values is inconsistent and does not support the existing practice in many laboratories of extrapolating gestation references from non-pregnant values. Laboratories should invest in deriving method-specific gestation reference intervals for their population.

The Clinical Value and Variation of Antithyroid Antibodies during Pregnancy

Antithyroid antibodies, which include thyroid-stimulating hormone receptor antibodies (TRAbs), thyroid peroxidase antibodies (TPOAbs), and thyroid globulin antibodies (TgAbs), are widely known for their tight association with thyroid autoimmune diseases. The variation in all three kinds of antibodies also showed different trends during and after pregnancy (Weetman, 2010). This article reviewed the the physiological changes, while focusing on the variation of thyroid antibodies concentration in women during and after pregnancy, and adverse consequences related to their elevation. Since abnormal elevations of these antithyroid antibodies may lead to adverse outcomes in both mothers and fetuses, special attention must be paid to the titer of the antibodies during pregnancy. The molecular mechanisms of the variations in those antibodies have yet to be explained. The frequency and timing of thyroid antibody measurement, as well as different reference levels, also remain to be elucidated.

Reference Intervals for Thyroid Hormones During the First Trimester of Gestation: A Report from an Area with a Sufficient Iodine Level

The physiological changes during pregnancy modulate the endocrine system. Therefore, both the American and the European thyroid associations recommend the use of local trimester-specific reference intervals. The purpose of this study was to establish the first trimester reference intervals for thyroid function tests in the central area of Iran. We examined 436 pregnant women in their first trimester of pregnancy, and 444 non-pregnant women in a cross sectional study. Serum levels of thyroid stimulating hormone (TSH), free thyroxin (FT₄), free triiodothyronine (FT₃), thyroid peroxidase antibody, urinary iodine concentration (UIC), and thyroid volume were measured for all subjects. The first trimester-specific reference intervals (2.5th-97.5th percentile) were determined for 185 pregnant women and 256 non-pregnant women with negative TPOAb, adequate iodine level (UIC≥150 μg/l in pregnant and UIC≥100 μg/l in non-pregnant women), and normal thyroid examination. We calculated multiples of the median (MoM) for TFTs to normalize the obtained data. The first trimester-specific reference intervals of serum TSH, FT₄, and FT₃ for pregnant women were 0.20-4.60 mIU/l, 9.0-18.02 pmol/l, and 3.40-5.64 pmol/l, respectively, while the corresponding figures for non-pregnant women were 0.59-5.60 mIU/l, 9.52-19.30 pmol/l, and 3.70-5.55 pmol/l, respectively. The first and 99th percentile MoM of TSH in pregnant women in their first-trimester was 0.06-4.62. The local normal reference ranges for the first trimester of pregnancy in central region of Iran were different from the ranges suggested by the ATA.

The prevalence of maternal hypothyroidism in first trimester screening from 11 to 14 weeks of gestation

AIM

The aim of this study was to determine the prevalence of maternal hypothyroidism in the first trimester from 11 to 14 weeks of gestation according to the American Thyroid Association (ATA) guidelines from 2017 and to compare the rates for singleton and twin pregnancies.

METHODS

A total of 4965 consecutive Caucasian singleton pregnancies and 109 Caucasian twin pregnancies were included in the investigation. Patients with a history of thyroid gland disorder were excluded. Subclinical maternal hypothyroidism was defined as a thyroid stimulating hormone (TSH) concentration above the 97.5^th percentile and free thyroxine (fT4) within the range of a reference population of women at 11-14 weeks of gestation. Overt maternal hypothyroidism was defined as a TSH concentration above the 97.5^th percentile and an fT4 below the 2.5^th percentile of the reference population.TSH, fT4, and anti thyroid peroxidase antibody (TPOAb) were measured by immunochemiluminescent assays on an 16200 Abbott Architect analyzer.

RESULTS

The prevalence of hypothyroidism for twin pregnancies was no higher than that for singleton pregnancies; 6.42% (7/109) vs. 5.32% (264/4965), respectively; P=0.61. All twin pregnancies were subclinical. Singleton hypothyroid pregnancies included 4.91% (244 cases) of subclinical and 0.41% (20 cases) of overt hypothyroidism. The prevalence of TPOAb positive hypothyroid women for twin pregnancies and singleton pregnancies was 71% (5/7) vs. 52% (137/264 cases), respectively but the differences were not statistically significant; P=0.31.

CONCLUSION

Each first trimester screening center should establish its TSH and fT4 reference ranges. Our center had higher upper reference limits of TSH than that of the universally fixed limit of 2.5 mU/L, which led to a lower measured prevalence of maternal hypothyroidism. A large number of hypothyroid women were TPOAb positive.