Reference values and the effect of clinical parameters on thyroid hormone levels during early pregnancy

Thyroid dysfunction is a common endocrine problem during pregnancy; correct diagnosis and appropriate treatments are essential to avoid adverse pregnancy outcomes. Besides, it is vital to identify and quantify the major risk factors for gestational thyroid dysfunction, including thyroid autoimmunity, human chorionic gonadotropin (HCG) concentration, body mass index (BMI) and parity. The study objective was to establish reference ranges during early pregnancy and to explore the relationship between risk factors and thyroid stimulating hormone (TSH), free thyroxine (FT4) and free triiodothyroxine (FT3).

To establish the reference ranges of thyroid hormone during early pregnancy in China and to identify the risk factors for thyroid dysfunction, woman in the first trimester of pregnancy (4-12 weeks gestation) were recruited. After excluding thyroid peroxidase antibody (TPO-Ab) positive and/or thyroglobulin antibody (TG-Ab) positive women, previous thyroid disease, a lack of iodine intake, reference values were calculated by 2.5th to 97.5th percentiles.

After exclusion of TPO-Ab and/or TG-Ab positive women, reference values were as follows: TSH, 0.11-3.67 mIU/l; FT3, 3.19-5.91 pmol/l; FT4 10.95-16.79 pmol/l. Higher BMI was associated with lower FT4 concentrations (P=0.005). In multiple regression analysis, TSH was significantly and positively associated with TG (P=0.03). Maternal parity and maternal age may be risk factors for the abnormal thyroidal response to hCG concentrations.

Our study defined first trimester-specific reference ranges for serum TSH, FT4, FT3 in a Chinese population, and demonstrated that BMI ≥23kg/m2, maternal parity ≥3 and maternal age ≥30 years may increase the risk of thyroid dysfunction.