Background: Normal maternal thyroid function is essential for optimal fetal neurological development. However, targeted screening for thyroid dysfunction in the first trimester of pregnancy, together with the criteria that should be applied, remains controversial.
Aim: To determine the efficacy of targeted screening for discovering new cases of thyroid dysfunction in pregnancy.
Method: All women who delivered their babies at Imperial College Healthcare NHS Trust between 1/3/17 and 31/12/17 and had thyroid function tests checked during pregnancy were retrospectively reviewed. Criteria for TFT testing include: diabetes; other endocrine disorders; drugs that affect the thyroid; symptoms of thyroid disease (including hyperemesis); family history of thyroid disease. Local reference ranges for 1st trimester include: TSH 0.23.6 miU/l and fT4 9.915.2 pmol/l.
Results: Thyroid function was checked in 417 women in the first trimester (median TSH 1.14 iU/l, IQR 0.551.79 iU/l). Of these women, 157 women had no history of thyroid dysfunction. Recorded reasons for screening included: diabetes including gestational diabetes (n=51); other endocrine disorders (n=5); hyperemesis (n=25); other autoimmune disorder (n=3); other (n=9). No reason was recorded for 64 women (40.8%). In women without known thyroid dysfunction, no new cases of clinical or subclinical hypothyroidism were detected (median TSH 0.85 iU/l, IQR 0.451.38 iU/l). In 17 (10.8%) women, TSH measured <0.3, and of these 10 had a fT4 above the upper limit of the reference range. Nine women with low TSH had documented hyperemesis.
Discussion: In this multi-ethnic cohort, targeted screening identified no new cases of clinical or subclinical hypothyroidism in the 1st trimester. This would argue against testing for hypothyroidism in women without a history of thyroid dysfunction, unless there is a strong suspicion of thyroid disease. Larger numbers are needed to confirm this. Irrespective, clinicians must remain vigilant for cases of hyperthyroidism, especially as the symptoms overlap with those of hyperemesis within the first trimester.