Trimester-Specific Reference Intervals of Thyroid Function in Healthy Pregnant Women in Macau
Abstract
Background: To establish the trimester-specific reference intervals of thyrotropin (TSH), free thyroxine (FT4) and free triiodothyronine (FT3) in healthy pregnant women in Macau.
Methods: Serum samples were collected from 166 healthy pregnant Macau women since early pregnancy until the third trimester. The study was performed in Macau Kiang Wu Hospital from July 2020 to October 2021.Basic clinical and obstetrics data were gathered using questionnaires. Blood samples were sequentially collected from the pregnant women at the first (≤12weeks), the second (12-28 weeks) and the third (>28 weeks) trimesters, respectively.
Result: Reference intervals of TSH were 0.02~3.30mIU/L, 0.45~3.80mIU/L and 0.18~3.43mIU/L in three trimesters. For FT4, Reference intervals were 12.82~22.0pmol/L, 9.86~15.58pmol/L and 10.10~15.30pmol/L in three trimesters. For FT3, Reference intervals were 3.53~6.0pmol/L, 2.74~4.65pmol/L and 2.62~4.45pmol/L in three trimesters. The concentration of TSH was significant lower in the first trimester compared to the second and third trimesters of pregnancy (median o.89, 1.66, 1.41mIU/L); TSH concentration differences between trimesters are significantly different (p<0.05).FT4 values decreased with the progression of gestational period (median 16.60, 12.40, 12.15pmol/L). FT3 values decreased with the progression of gestational period (median 4.56, 3.65, 3.57pmol/L). FT4 and FT3 concentration had significantly differences between first and second trimesters (p<0.05), but had no significantly differences between second and third trimester (p>0.05).
Conclusion: This study is the first to establish trimester-specific reference intervals of TSH, FT4 and FT3 in healthy pregnant Macau women. Trimester-specific reference intervals may help in diagnosis and management of thyroid dysfunction during pregnancy which will prevent both maternal and fetal outcomes.
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Introduction
Disease of the thyroid gland affects about 5% of the general population, and predominantly affects females [1]. During pregnancy, the thyroid gland increase in size by 10% in iodine-sufficient countries but by 20% to 40% in areas of iodine deficiency. Production of the thyroid hormones, thyroxine (T4), and triiodothyronine (T3), increases by nearly 50%, in conjunction with a separate 50% increase in daily iodine requirement. Normal thyroid function is essential for fetal development. The fetus is totally dependent on maternal thyroxine supply during the first trimester and second trimester for normal development and nerves system maturation. Because the progression of pregnancy and fetal, neonatal and child health are dependent on adequate thyroid hormones supplementation throughout pregnancy.A deficiency or an excess of thyroid hormone can occur in pregnancy. Thyroid dysfunction during pregnancy is common,with a prevalence of 2-4%. The prevalence of overt hyperthyroidism is approximately 0.1-0.4%, subclinical hyperthyroidism about 3.3% [2], overt hypothyroidism about 0.3%, and subclinical hypothyroidism may reach 2.5% or more, thyroid nodules about 3-21%[3].
Conclusion
We found that the upper limit of serum TSH reference in the first trimester and second trimester were much higher than 2.5mIU/L and 3.0mIU/L in our population. On the other hand, the upper limit of serum TSH references in trimesters of our study was lower than 4mIU/L which proposed by ATA 2017 guideline. The trimester-specific reference ranges for serum TSH during pregnancy obtained from similar populations and maternal iodine status, but using different laboratory assay method were inconsistent. In conclusion, we established the trimester-specific reference intervals of TSH, FT4 and FT3 in TPOAb negative pregnant women from a tertiary care centre in Macau. In our opinion, measurement of TSH, FT4 and TPOAb in the systematic screening for thyroid dysfunctions in pregnancy is beneficial and it can help decide whether women should be treated with levothyroxine or not.