We have performed a survey of the incidence of thyroid disorders during pregnancy in local situation of southern China. The results of the survey indicated that thyroid dysfunction is a common disorder among local pregnant women. In fact, at least one in seven pregnant women suffers from thyroid disorder. The results of study are approximately consistent with the latest meta-analysis results that showed the prevalence of overt hypothyroidism, isolated hypothyroxinemia, and sub-clinical hyperthyroidism to be 0.5%, 2.05%, and 2.18% [18]. But compared with the meta-analysis which showed the prevalence of subclinical hypothyroidism and hyperthyroidism to be 3.47% and 0.91%, the subclinical hypothyroidism revealed the higher frequency (6.68%) and the hyperthyroidism revealed the lower frequency (0.45%) in our study. In Denmark, the incidence rates of subclinical hypothyroidism and hyperthyroidism were 5.3% and 1.6%, respectively [19]. In previous studies of China, the prevalence of subclinical hypothyroidism and clinical hyperthyroidism was reportedly 5.27% and 0.4%, respectively [17]. We believed the results might be due to the iodine intake of residents in different area, and classification of thyroid status by different criteria, the other reason for this phenomenon may was that we did not refer to the status of thyroid peroxidase antibody (TPO-Ab) in our study. We also determined the prevalence of gestational transient thyrotoxicosis along with endocrinologists (3.26%). It was occasionally difficult to distinguish GTT from hyperthyroidism, which be distinguished by several clinical and laboratory differences [5]. GTT was related to the high level of hCG (Human chorionic gonadotropin) secreted by the placenta, which was often manifested as hyperemesis and not ordinarily require intervention. Its appearance was a physiological process, not an immune abnormality; its thyrotropin receptor (TRAb) often showed negative results [20].
In the study reported herein, there were various impacts on adverse pregnancy outcomes in different thyroid dysfunction groups. The pregnant women with hypothyroidism showed more common preeclampsia and postpartum hemorrhage. Compared with control group, overt hypothyroidism increased the incidence rates of spontaneous abortion, premature delivery, therapeutic abortion related to fetal diseases and macrosomia, and overt hyperthyroidism increased the incidence rates of premature delivery, therapeutic abortion related to fetal diseases, GDM and preeclampsia. However, the difference was not significant (P > 0.05). We considered that the main reasons might be related to using levothyroxine or propylthiouracil (PTU) once the hypothyroidism or hyperthyroidism was diagnosed. Many of previous studies showed that untreated hypothyroidism and hyperthyroidism can cause serious adverse consequences for the pregnant women and fetal [6, 12, 21–23], however, if the maternal hyperthyroidism was adequately treated, prognosis was well [5, 23]. According to the existing guidelines [7], all participants identified as overt hyperthyroidism and hypothyroidism were treated immediately and kept TSH in an ideal range continuously in our study. Our results reflect the obstetrical outcomes of pregnant women with treated hypothyroidism and hyperthyroidism.
Many existing studies had demonstrated that subclinical hypothyroidism also had adverse effects on pregnancy outcomes, for example preeclampsia, GDM, spontaneous abortion, premature delivery, although the effects were less than overt hypothyroidism [24–26]. But in Mannisto’s study, the results showed that maternal subclinical hypothyroidism was not related to adverse pregnant outcomes [27]; and Cleary-Goldman also reported similar results [14]. In our study, the incidence of preeclampsia, GDM, PROM, therapeutic abortion related to fetal diseases were increased. But the incidence of spontaneous abortion, macrosomia, PPH, premature delivery breech presentation, intrauterine fetal death did not show significant increasing in subclinical hypothyroidism group than control group. Inconsistent research conclusions might be related to the timing of levothyroxine use, but we tend to think that the effects of subclinical hypothyroidism on pregnancy outcomes were complex and worthy of further study.
There was no dispute that subclinical hyperthyroidism and gestational transient thyrotoxicosis had little adverse effects on pregnancy outcome and obstetrical complications [20, 21, 28], our results are also consistent with the conclusions.
It is worth noting that the effects isolated hypothyroxinemia on gravidas and fetus. At present, there were few studies on the relationship between the isolated hypothyroxinemia and adverse pregnant outcomes, and the results of studies were controversies [29–31]. Some studies have reported increased risk of premature delivery, fetal distress, and premature rupture of membranes and higher mean birth weight in isolated hypothyroxinemia [32]. However, other studies have reported no increased risk of alteration in the offspring of women with isolated hypothyroxinemia [33]. In our study, the incidences of spontaneous abortion, premature delivery, GDM, preeclampsia, macrosomia, breech presentation were significantly higher in isolated hypothyroxinemia than those euthyroidism (P < 0.05). Our results played a role in complementing clinical data for the study of isolated hypothyroxinemia.
As for the treatment of hypothyroxinemia, there was still no consensus in the guidelines [7, 16], so that in the face of patients with isolated hypothyroxinemia, the decision to use levothyroxine often based on clinician’s experience or patient’s wishes rather than objective evidence. There was a large pregnant population suffered from isolated hypothyroxinemia if calculated by the incidence rate of 1.95%. Therefore, the significance of levothyroxine in isolated hypothyroxinemia urgently needs to be confirmed. The results of our study suggested that it was not significant improved on pregnancy outcomes and obstetrical complications by using levothyroxine (P > 0.05), although the incidence rate of spontaneous abortion, premature delivery and preeclampsia is lower in isolated maternal hypothyroxinemia group than control group.