Each first trimester screening center should be aware of which type of immunoassay their laboratory uses. TSH reference ranges in women during the first trimester of pregnancy are lower than those for general population. Twin pregnancies have lower TSH than singleton pregnancies.
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.
The aim was to assess the proportion of women requiring a referral to an endocrinologist when carrying out routine screening for maternal thyroid disease simultaneously with screening for Down syndrome. Between November 2009 and September 2015, women having a first trimester combined test were screened; those symptomatic or being investigated for thyroid problems were excluded. In the first half of the period the blood sample was tested for anti-thyroid peroxidase (TPO) antibodies, serum thyroid stimulating hormone (TSH), and free thyroxine (fT4); thereafter fT4 was not used. Women with at least one analyte outside the range (TPO more than 5.6 kU/L, TSH 0.35–4.94 mU/L, fT4 9.0–19.1 pmL/L) were referred to an endocrinologist for counseling. Of 10,052 women screened, 1190 (11.8%) had TSH, fT4, or antibody levels outside the range; 13.0% when fT4 was used and 10.4% thereafter. Most of these positive results were due to a single abnormal marker. There was a much higher positive rate in multiple pregnancies (27.3% compared with 11.5% in singletons;
P
< 0.0001). There was an association between positivity and maternal age (
P
< 0.0001), a higher rate in the small ethnic group of Asians (
P
< 0.0005) and a lower rate among smokers (
P
< 0.02) and following assisted reproduction (
P
< 0.05). Routine antibody and hormone testing at the same time as the combined test generated a large number of referrals but this did not overload the services.
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