Reference Values for TSH and Free Thyroid Hormones in Healthy Pregnant Women in Poland: A Prospective, Multicenter Study.

OBJECTIVES The diagnosis and treatment of thyroid diseases in pregnant women remains a challenge. Various medical associations recommend establishing the reference intervals for thyroid hormones by a local laboratory. Considering differences within geophysical, socioeconomic conditions, and iodine prophylaxis in various populations, it is advisable to assess reference intervals for thyroid hormones specific to a region of residence. The objective was to assess trimester-specific reference intervals for TSH, fT3, and fT4 for pregnant women in the Polish population. METHODS AND RESULTS We conducted a prospective study in 4 centers representing different regions of Poland (Krakow, Warsaw, Poznan, and Bialystok). Our study included consecutive, healthy pregnant women (172 patients), with an age range of 27-47 years. All women had a negative history for thyroid diseases, normal thyroid peroxidase antibody levels, and proper iodine prophylaxis. All newborns had TSH levels in the appropriate reference range. Serum TSH, fT3, fT4, and thyroid-peroxidase antibodies were measured in each trimester. The reference intervals were calculated using the percentile method, as recommended by the International Federation of Clinical Chemistry. The reference values calculated were 0.009-3.177, 0.05-3.442, and 0.11-3.53 mIU/L for TSH; 3.63-6.55, 3.29-5.45, and 3.1-5.37 pmol/L for fT3; and 11.99-21.89, 10.46-16.67, and 8.96-17.23 pmol/L for fT4 in consecutive trimesters of pregnancy. Reference intervals for pregnant women when compared to the general population showed a lower concentration of TSH in every trimester of pregnancy and lower fT4 in the 2nd and 3rd trimesters. CONCLUSIONS Using appropriate trimester-specific reference intervals may improve care of pregnant women by preventing misdiagnosis and inadequate treatment.