Ethnic differences in TSH but not in free T4 concentrations or TPO antibodies during pregnancy

To describe the TSH, free T4 and thyroid peroxidase antibody (TPO-Ab) concentrations during pregnancy among four ethnic groups and to determine reference values for these parameters during normal pregnancy.

Cross-sectional study of a cohort of 3270 pregnant women living in the city of Amsterdam. Blood samples were drawn at first booking in the first or second trimester. TSH, free T4 and TPO-Ab concentrations were determined. Four ethnic groups were studied: Dutch, Surinam, Turkish and Moroccan.

Plasma TSH increased and free T4 decreased from the first to the second trimester of pregnancy for all the ethnic groups. Ethnic differences were observed in TSH concentrations, with Dutch females having the highest TSH levels compared to the other three ethnic groups. The median TSH difference was 0.16 mU/l between the Dutch and Moroccan women, 0.15 mU/l between the Dutch and Surinam women and 0.10 mU/l between the Dutch and Turkish women. These could not be explained by differences in age, parity and current smoking status. No differences were seen in free T4 concentrations between the four ethnic groups. The prevalence of TPO-Ab was comparable across the ethnic groups (about 6% in each); the concentration of TPO-Ab was also comparable among the ethnic groups. The Dutch women had a higher lower-limit (2.5 percentile) of the TSH reference range than the Surinam, Turkish and Moroccan women, ranging from 0.14 mU/l for the Surinam and Moroccan to 0.27 mU/l for the Dutch women.

The increase in TSH and decrease in free T4 values during pregnancy correspond to previous reported studies. Pregnant Dutch women had consistently higher TSH values than the ethnic group, but corresponding free T4 levels and TPO-Ab did not differ between these ethnic groups.