A Population-based Cohort Study: Maternal Thyroid Hormone Levels During Pregnancy And Pregnancy Outcomes

Background Maternal thyroid function in pregnancy is crucial to mother andpregnant outcome. But the study results of many maternal thyroid function weredifferent and contradictory. Our findings based on cohort study were more credible.Objectives To describe maternal thyroid founction of first and second trimester fromMa’anshan city, Anhui province. To observe the dynamic change of the level ofmaternal thyroid hormones in pregnancy, to analyze the effect factors and theassociation between the maternal founction of first and second trimester and adversepregnant outcome.Methods In the population-based cohort study, our subjects were pregnant womenwho had their first antenatal examination and signed informed consent voluntarily atMa’anshan Maternity and Child Care Centers from October2008to October2010. Theinclusion criterias were:①no living history in iodine deficiency area;②no thyroiddisease history and the family history;③no history of autoimmune disease;④nohistory of mental illness. The information about sociodemographic characteristics,lifestyle behavior, history of pregnancy and nutrition are collected from Maternal andChild Health Record Form filled by pregnant women with the guidance of trainingdiretctor. Doctors of clinical laboratory collected5ml Peripheral venous blood frompregnant women when they had their first prenatal care and preserved in lowtemperature environment after centrifugation. Using Luminescence Immunoassay to measure the level of maternal thyroid hormones after pregnant women gave birth totheir children. The measurement indexes are thyroid stimulating hormone (TSH), freethyroxine (FT4) and thyroid peroxidase antibody (TPOAb). Gestional specific thyroidreference ranges were made by percentile method through SPSS13.0. To evaluatematernal thyroid founction and analyze the effects from maternal sociodemographicvariable and lifestyle behavior. To collect pregnant outcome and some relatedinformation by Maternal and Child Health Record Form and Population-based BirthDefect Registration Card. The differences of birth index (such as birth height, weight,head circumference and chest circumference) with different maternal thyroid founctionof first and second trimester were analyzed by Student t-test. Chi-square andnon-condition logistic regression were performed to analyze the association betweenmaternal mild hypothyroidism in pregnancy and pregnant outcome.Results Sociodemographic and other related informations were gathered from5089pregnant women of Ma’anshan city, Anhui province from October2008to October2010. There were4805valid questionnaires and4602serum samples. The finalsubjects were3387pregnant women while ruled out pregnant women whio got twins,thyroid disease, autoimmune disease or serious illness history and no gestational ageand whose serum weren’t used to do thyroid hormones tests.To access maternal thyroid hormones according to percentile method, there were789pregnant women who got mild hypothyroidism (subclinical hypothyroidism,hypothyroxinemia, TPOAb poistive) in3387pregnant women from Ma’anshan city,Anhui province. Maternal TSH levels went up and maternal FT4levels went down fromgestational age of5～24. Maternal TSH levels of second trimester were higher thanmaternal TSH levels of first trimester, maternal FT4levels of second trimester werelower than maternal FT4levels of first trimester. Maternal FT4and TSH, TPOAb levelsof first and second trimester were respectively negative correlated, maternal TSH and TPOAb levels of first and second trimester were positive correlated. Pregnant womenwho were younger and had drinking history half a year ago had high level of TSH(P<0.05). Pregnant women who were older and overweight or obese, urban registeredresidence and average level of family per capita monthly income had low level of FT4(P<0.05).To analyze the3387data with pregnant outcome, TPOAb positive pregnant womenhad higher incidence of subclinical hypothyroidism (2=27.656, P<0.001), thedifferences of incidence of maternal hypothyroxinemia in TPOAb positive pregnantwomen were not statistically significant. To explore the influence that maternal thyroidfounction of first and second trimester make on the birth index of offspring. The valueof birth weight, body height and head circumference of offspring in maternalhypothyroxinemia group were greater than that of normal maternal thyroid functiongroup. After adjusted for confounding factors (such as maternal age, registeredresidence and pregnant times), the association between maternal mild hypothyroidismof first and second trimester and adverse pregnant outcome was to be analyzed.Pregnant women who had subclinical hypothyroidism (95%CI:1.09～12.41) andTPOAb positive (95%CI:1.27~12.27) in first and second trimester were more likely tohave stillbirth outcome, the OR were3.67and3.94respectively. Pregnant women whogot TPOAb positive in first and second trimester increased the risk of birth defect(OR=1.50,95%CI：1.03～2.20).Conclusions The levels of maternal thyroid hormones of first and second trimesterwere changeable and influenced by many factors. Maternal thyroid disfunction couldrise the risk of adverse pregnant outcome and birth defect. It was helpful to establishgestational specific thyroid function reference range and do screening and treatment topregnant women who got thyroid disfunction as soon as possible.