Maternal thyroid function during the first half of pregnancy

Objective To analyze the changes of thyroid function of healthy primipara before 20 weeks of gestation to establish normal gestational age specific reference interval of thyroid hormones, and to investigate the prevalence of maternal thyroid disorders during the first half of pregnancy, Methods A total of 1605 healthy primipara without risk factors of thyroid diseases before 20 gestational weeks and 200 non-pregnant healthy women who accepted preconception care in Beiiing Friendship Hospital from September 2010 to June 2011 were tested for serum thyroidstimulating hormone （TSH）, free thyroxine （FT4） and thyroid peroxidase antibody （TPOAb） by chemiluminometric immunoanalysis. One thousand two hundred and fourty-three pregnant women among them with negative thyroid antibooly and without previons thyroid diseases were selected as the standard population for normal interval. Gestational age-specific percentile categories for TSH and FT4 were calculated. The prevalence of maternal thyroid disorders was examined by gestational age- specific intervals. Results （1） Compared with non-pregnant women, the median value of serum TSH in pregnant women decreased by 29.56% to the value of 0.91 mU/L; while that of FT4 rose by 7.79% to the value of 11.33 pmol/L before 12 weeks; and TSH increased while FT4 decreased during 13 to 20 weeks. （2） The median values and reference intervals （2.5th percentile, 97.5th percentile） for TSH were I. 59 mU/L （0.15 mU/L, 5.19 mU/L） in no-pregnant women, 1.12 mU/L （0.03 mU/L, 3.67 mU/L） at 8-12^＋6 gestational weeks, 1.21 mU/L （0.05 mU/L, 3.74 mU/L） at 13-16^＋6 gestational weeks, 1.50 mU/L （0.31 mU/L, 4.33 mU/L） at 17-19^＋6 gestational weeks; and the median values and reference intervals （2. 5th percentile, 97. 5th percentile） for FT4 were 9. 91 pmol/L （6.69 pmol/L, 14.03 pmol/L）, 10.68 pmol/L （7.98 pmol/L, 18.66 pmol/L）, 10.04 pmol/L （6.18 pmol/L, 16.22 pmol/L）, 9.40 pmol/L （6.44 pmol/L, 13.51 pmol/L） respectively. （3） According to gestational age-specific reference intervals, the general prevalence of maternal thyroid disorders, including hyperthyroidism, hypothyroidism, subclinical hypothyroidism and hypothyroinemia, was 3.55% （57/1605）. At 8-12^＋6 gestational weeks, 13-16^＋6 gestational weeks and 17-19^＋6 gestational weeks, the occurrence of hyperthyroidism was 0.00%, 0.13% and 0.00% ; that of hypothyroidism was 0.00%, 0. 13% and 0.00% ; the incidence of subclinical hypothyroidism was 3.60%, 2.76% and 3.00% ; the occurrence of hypothyroxinemia was 0.16%, 0： 26% and 0.86%, respectively. The positive rate of TPOAb at 8 12^＋6 , 13-16^＋6 and 17-19^＋6 gestational weeks were 22. 91% （140/611）, 16. 56% （126/761） and 15.45%（36/233）, and the total positive rate of TPOAb was 18.82% （302/1605）. The median level of TPOAb was 38. 90, 41.87 and 39.10 mU/L, respectively. Conclusions Before 20 gestational weeks, specific changes occur in maternal thyroid function. TSH level decreases during 8 to 12 gestational weeks, and then increases gradually; while FT4 level increases during 8 to 12 weeks, and then decreases gradually. Thyroid dysfunction during pregnancy is common and subclinical hypothyroidisum is the leading problem in thyroid disorders. Screening for thyroid function during early pregnancy is suggested. Key words: Pregnancy;  Thyrotropin;  Thyroxine;  Reference values;  Hypothyroidism;  Prevalence