Who should we screen for thyroid dysfunction during pregnancy?

Topic: THYROID AND PREGNANCY

Title: Detection of thyroid dysfunction in early pregnancy: universal screening or targeted high-risk case finding?

Authors: Vaidya B, Anthony S, Bilous M, Shields B, Drury J, Hutchison S, Bilous R.

Reference: Journal of Clinical Endocrinology and Metabolism 92: 203-207, 2007

Summary

Background

Maternal subclinical hypothyroidism in pregnancy is common (about 2.5%). Screening has not been recommended but targeted case finding has been advocated in women with a thyroid history, autoimmune disorder, or family history of thyroid disorder.

Purpose

To assess efficacy of targeted high-risk case finding compared to screening of a total population in identifying women with thyroid dysfunction during early pregnancy.

Patients and Methods: In a prospective single-centre cohort study, serum TSH, FT 4 and FT 3 were measured in 1.560 women at a median of 9 weeks gestation. TPO antibodies were measured in 1.327 (85%). The high-risk group (with a personal history of thyroid disease or other autoimmune disorder, or a family history of thyroid disorder) numbered 413 women (26.5%) and the low risk group 1.147 (73.5%).

Results

Forty women (2.6%) had a raised (> 4.2 mIU/liter) TSH: 6.8% in the high risk group versus 1% in the low risk group (p < 0.0001). Presence of personal history of thyroid disease (RR: 12.2; p < 0.0001), other autoimmune disorders (RR: 4.8; p = 0.016) and family history of thyroid disorders (RR: 3.4; p < 0.0001) increased the risk of finding high serum TSH. However, 12/40 women with raised TSH (30%) were in the low risk group.

Conclusions

Screening for thyroid dysfunction in the high risk group alone would miss about 30% of women with subclinical or overt hypothyroidism.

Commentary

Subclinical hypothyroidism in gestation is associated with adverse obstetric outcomes including premature delivery, preeclampsia, breech delivery and increased fetal mortality. The presence of TPO antibodies is associated with an increased risk of miscarriage and the development of postpartum thyroid dysfunction. In addition, and of critical importance to the child, gestational subclinical hypothyroidism is now accepted as a significant risk factor for impairment of neuropsychological performance in childhood. These facts are thought by some to underpin the necessity for routine thyroid screening in early gestation with the aim of treating women with thyroid dysfunction and, thereby, preventing the outcomes referred to above.

However, recent guidelines from the Endocrine Society as well as the American Thyroid Association and The American Association of Clinical Endocrinology have not recommended screening, as there is no evidence base in terms of a prospective randomised trial to justify this approach. This is also the view of The American College of Obstetrics and Gynecology. Instead, these organisations have suggested targeted case finding using the criteria stated in this study.

The importance of present study is that, using the targeted criteria, 30% of women with significant thyroid impairment would not come to medical attention as having thyroid dysfunction. Another feature is the finding that 1.6% of women had a low FT 4 despite a normal TSH and negative TPO antibodies. As placental transfer of T 4 is critical for fetal neuronal maturation, this aspect deserves further study.

Although an incidence of high TSH of around 2.5% should justify routine screening, the implications (clinical & cost) mean that we must await the results of two prospective randomised trials (one in the UK and one in the USA) designed to evaluate the effect of maternal levothyroxine therapy given to women found to have a high TSH or low FT 4 in early gestation on the intelligence quotient of their child before further progress is made. ( Summary and commentary prepared by John Lazarus )

Present summary and commentary are related to Chapter N- 14 of TDM