Maternal thyroid parameters in pregnant women with different ethnic backgrounds: do ethnicity-specific reference ranges improve the diagnosis of subclinical hypothyroidism?

Abstract

Objective

Guidelines on the management of thyroid dysfunction during pregnancy have recently been updated and, for the diagnosis of subclinical hypothyroidism (SCH), a TSH upper reference limit (cutoff) of 4.0 mIU/L has been proposed when no institutional values are available. It is also suggested that serum TSH and thyroid autoimmunity (TAI) may be different according to the ethnic background of the women. We therefore determined the prevalence of TAI and SCH in pregnant women with different ethnic backgrounds and, to define SCH, we used different first trimester TSH upper reference cutoffs (institutional, ethnicity-specific, 2.5 mIU/L (Endocrine Society) and 4.0 mIU/L (American Thyroid Association)).

Design

Cross-sectional data analysis of 1683 pregnant women nested within an ongoing prospective database of pregnant women.

Method

The study was performed in a single centre in Brussels, Belgium. During the first antenatal visit, thyroid peroxidase antibodies (TPO-ab), thyroid-stimulating hormone (TSH) and free T4 (FT4) were measured and baseline characteristics recorded. Data from 481 women with Sub-Saharan (SaBg; 28.6%), 754 North-African (NaBg; 44.8%) and 448 Caucasian (CaBg; 26.6%) backgrounds were analysed. For the calculation of TSH reference ranges, women with TAI, outliers, twin and assisted pregnancies were excluded.

Results

The prevalence of TAI was significantly lower in the SaBg group than in NaBg and CaBg groups (3.3% vs 8.6% and 11.1%; p<0.001 respectively). Median TSH was significantly lower in SaBg and NaBg groups as compared with the CaBg group (1.3 and 1.4 vs 1.5 mIU/L; p=0.006 and 0.014 respectively). The prevalence of women with SCH was comparable between all groups when 2.5 mIU/L was used as cutoff, but when 4.0 mIU/L or the institutional cutoff (3.74 mIU/L) was used, it was significantly higher in the CaBg group vs the NaBg group (5.4% vs 2.1% and 7.1 vs 3.3%, p=0.008 and 0.013 respectively). The use of ethnicity-specific cutoffs did not change the prevalence of SCH as compared to the use of institutional cutoffs. However, when these cutoffs were used the prevalence of SCH reduced by >70% (4.5% instead of 16.7%; p<0.001) relative to the 2.5 mIU/L cutoff,.

Conclusions

Pregnant women with a Sub-Saharan African background had a lower prevalence of TAI and TSH levels as compared with women from other backgrounds. The use of ethnicity-specific TSH cutoffs in early pregnancy was not more specific for the diagnosis of SCH as compared to the use of the institutional cutoff.

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