Effects of different reference intervals, maternal age and thyroid peroxidase antibody on incidence of gestational thyroid diseases

Objective To determine the reference intervals for thyroid function tests during the second half of pregnancy (20-40 gestational weeks), and to assess the relationship between thyroid peroxidase antibody (TPOAb) levels and the incidence of gestational thyroid diseases. Methods Levels of thyroid stimulating hormone (TSH), free thyroxine (FT4), TPOAb and urinary iodine excretion were determined in 4 729 pregnant women, who received prenatal health care at First Affiliated Hospital of Nanjing Medical University from July 2011 to August 2013. Among these women, 2 568 were selected using the recommendations of the American National Academy of Clinical Biochemistry, and were divided into five groups according to their gestational age: ≥20 to <24 weeks (682 cases), ≥24 to <28 weeks (1 322 cases), ≥28 to <32 weeks (178 cases), ≥32 to <36 weeks (185 cases) and ≥36 to≤40 weeks (201 cases). Reference intervals of thyroid function tests in the second half of pregnancy were calculated. The reference values of thyroid functions in different gestational weeks were compared, and the reference intervals of thyroid functions in the second half of pregnancy were determined. The effects of maternal age and positive TPOAb on gestational thyroid diseases were analyzed. A non-parametric test, analysis of variance or Chi-square test was used for statistical analysis. Results (1) Reference intervals for maternal thyroid function in the second half of pregnancy in our hospital were established [TSH: 0.65-5.27 mU/L and FT4: 8.74-14.84 pmol/L]. (2) The percentage of thyroid diseases was higher using the non-pregnancy reference intervals (TSH: 0.27-4.20 mU/L and FT4: 12.00-22.00 pmol/L) than using the pregnancy reference intervals [64.0% (3 025/4 729) vs 16.1% (763/4 729), χ2=47.465, P 30 years was higher than in those aged ≤30 years [0.7% (10/1 377) vs 0.3% (10/3 352), χ2=4.257; 11.7% (161/1 377) vs 8.0% (268/3 352), χ2=16.102; both P<0.05]. The incidence of clinical hypothyroidism and clinical hyperthyroidism in TPOAb positive women was higher than that in TPOAb negative women [2.7% (9/335) vs 0.3% (11/4 394), χ2=44.009; 3.9% (13/335) vs 1.2% (52/4 394), χ2=16.784; both P<0.01]. Conclusions The established pregnancy-specific reference ranges of thyroid function tests can reduce the missed diagnosis and misdiagnosis of gestational thyroid diseases. Maternal age >30 years and positive TPOAb may increase the risk of gestational thyroid diseases. Key words: Pregnancy complications; Thyroid diseases; Incidence; Iodide peroxidase; Autoantibodies; Reference values; Age factors