Gestation specific reference intervals for thyroid function tests in pregnancy.

Abstract

Thyroid function tests are frequently assessed during pregnancy to evaluate thyroid dysfunction or to monitor pre-existing thyroid disease. However, using non-pregnant reference intervals can lead to misclassification. International guidelines recommended that institutions should calculate their own pregnancy-specific reference intervals for free thyroxine (FT4), free triiodothyronine (FT3) and thyroid-stimulating hormone (TSH). The objective of this study is to establish gestation-specific reference intervals (GRIs) for thyroid function tests in pregnant Turkish women and to compare these with the age-matched non-pregnant women. Serum samples were collected from 220 non-pregnant women (age: 18-48), and 2460 pregnant women (age: 18-45) with 945 (39%) in the first trimester, 1120 (45%) in the second trimester, and 395 (16%) in the third trimester. TSH, FT4 and FT3 were measured using the Abbott Architect i2000SR analyzer. GRIs of TSH, FT4 and FT3 for first trimester pregnancies were 0.49-2.33 mIU/L, 10.30-18.11 pmol/L and 3.80-5.81 pmol/L, respectively. GRIs for second trimester pregnancies were 0.51-3.44 mIU/L, 10.30-18.15 pmol/L and 3.69-5.90 pmol/L. GRIs for third trimester pregnancies were 0.58-4.31 mIU/L, 10.30-17.89 pmol/L and 3.67-5.81 pmol/L. GRIs for TSH, FT4 and FT3 were different from non-pregnant normal reference intervals. TSH levels showed an increasing trend from the first trimester to the third trimester, whereas both FT4 and FT3 levels were uniform throughout gestation. GRIs may help in the diagnosis and appropriate management of thyroid dysfunction during pregnancy which will prevent both maternal and fetal complications.