Context: Abnormal maternal thyroid function during pregnancy is associated with various complications. International guidelines advocate the use of population-based trimester-specific reference ranges for thyroid function tests. When unavailable, an upper TSH limit of 2.5 for the first trimester and 3.0 mU/L for the second and third trimesters is recommended. Although interindividual differences in thyroid function tests can partially be explained by ethnicity, data on the influence of ethnicity on TSH and free T4 reference ranges during pregnancy are sparse.
Design: Serum TSH, free T4, T4, and TPO-antibody levels were determined during early pregnancy in 3944 women from the Generation R study, Rotterdam, The Netherlands.
Results: The study population consisted of 2765 Dutch, 308 Moroccan, 421 Turkish, and 450 Surinamese women. Mean TSH levels were higher in Dutch and Turkish women than in Moroccan or Surinamese women (1.50-1.48 vs 1.29-1.33 mU/L; P < .01). Although no differences in free T4 were seen, T4 was lowest in Dutch women (142 vs 150-156 nmol/L; P < .01). Turkish women had the highest frequency of TPO-antibody positivity (9.3% vs 5.0-5.8%; P < .05) and of elevated TSH levels in the second trimester (11.0% vs 3.8-7.3%; P < .01). A comparison of disease prevalence between a population-based vs an ethnicity-specific reference range changed the diagnosis for 18% of women who were initially found to have abnormal thyroid function test results.
Conclusions: We show ethnic differences in serum TSH, T4, and TPO-antibody positivity and found significant diagnostic discrepancies depending on whether population or ethnicity-specific reference ranges were used to diagnose thyroid disease.