Thyroid Function During Pregnancy in A Multiethnic Population in Norway

Line Sletner; Anne Karen Jenum; Elisabeth Qvigstad; Sara Salehi Hammerstad

doi:10.1210/jendso/bvab078

Thyroid Function During Pregnancy in A Multiethnic Population in Norway

J Endocr Soc. 2021 May 4;5(7):bvab078. doi: 10.1210/jendso/bvab078. eCollection 2021 Jul 1.

Authors

Line Sletner^{1

2}, Anne Karen Jenum³, Elisabeth Qvigstad^{4

5}, Sara Salehi Hammerstad^{6

7}

Affiliations

¹ Department of Pediatric and Adolescents Medicine, Akershus University Hospital, PB 1000, 1478 Lørenskog, Norway.
² Institute of Clinical Medicine, Campus Ahus, University of Oslo, Norway.
³ General Practice Research Unit (AFE), Department of General Practice, Institute of Health and Society, Faculty of Medicine, University of Oslo, PB 1130 Blindern, 0318, Oslo, Norway.
⁴ Department of Endocrinology, Morbid Obesity and Preventive Medicine, Oslo University Hospital, Aker, PB 4950, 0424 Oslo, Norway.
⁵ Institute of Clinical Medicine, University of Oslo, Norway.
⁶ Department of Pediatric, Oslo University Hospital, Ullevål, PB 4950, 0424, Oslo, Norway.
⁷ Specialist Center Pilestredet Park, Pilestredet Park 12.A 0176 Oslo, Norway.

Abstract

Context: Ethnic differences in thyroid function during pregnancy have been reported. However, it is unclear if this is equally valid across ethnic groups within multiethnic populations.

Objective: We aimed to assess ethnic differences in thyrotropin (TSH) and free thyroxine (FT4), and the prevalence of thyroid dysfunction and thyroid autoimmunity during pregnancy.

Methods: In a population-based cohort of 785 pregnant women in Oslo, Norway, TSH, FT4, and thyroid peroxidase antibodies (TPO Abs) were measured twice: at gestational week (GW) 15 and 28, and urine iodine concentration at GW 15. Associations were assessed using multivariate linear regression.

Results: We found ethnic differences in TSH levels at both time points, but not for fT4. South Asians had 0.42 mU/L (95% CI, 0.20-0.64) higher TSH than Europeans in GW 15. This difference persisted after adjusting for covariates (including TPO Ab positivity and iodine status), and increased further as pregnancy progressed. In contrast, East Asians had the lowest TSH. No new cases of overt hypothyroidism were detected in early pregnancy, but subclinical hypothyroidism was found in 6.6% among all, highest in South Asians (14.2%). Hyperthyroidism early in pregnancy was observed in 3.7% (almost all subclinical), highest in East Asians (11.9%). The prevalence of TPO Ab positivity was 4%, highest in South Asians (8%).

Conclusion: In a multiethnic population of presumably healthy women, we found ethnic variations in TSH but not FT4 levels throughout pregnancy. South Asians had higher TSH and more subclinical hypothyroidism, not explained by their higher prevalence of TPO Ab positivity. Larger studies are needed to define ethnic- and trimester-specific reference ranges in pregnancy.

Keywords: ethnic differences; hyperthyroidism; subclinical hypothyroidism; thyroid and pregnancy; thyroid epidemiology.