Endocrine Abstracts (2004) 7 P243

Controlled anti-natal thyroid screening study (CATS) - early results

JH Lazarus1, AB Parkes1, IL Taylor1, PPA Smyth2, R John3 & N Wald4

1Department of Medicine, University of Wales College of Medicine, Cardiff, UK; 2Department of Medicine, University College, Dublin, Ireland; 3Medical Biochemistry, University of Wales College of Medicine, Cardiff, UK; 4The Royal London & St Bartholemew's Hospital, London, UK.

This study aims to recruit 22,000 women in early gestation to screen for thyroid failure. The trial is randomised such that half the women are being tested in early gestation (group1) while the remainder (group 2) are tested after delivery. As T4 is important for the developing foetal brain, the IQ of the children from group 1, whose mothers will have been treated with T4, is expected to be higher than that from group 2.

Women are entered on the basis of either a serum TSH greater than the 2.5th percentile or a free T4 less than the 2.5th percentile. We expected to find a significant number of women with both an elevated TSH plus a low free T4. However, of the first 2713 women recruited only 6 had abnormal free T4 and TSH while 49 had raised TSH and 44 a reduced free T4. Further, free T4 levels were significantly lower (p < 0.05) in the women with elevated TSH when compared with a random group of control women. Preliminary observations suggest that, whilst only 10% of women with low free T4 showed elevated antithyroid peroxidase antibodies, 50% of the women with raised TSH were antibody positive.

The results so far cast doubt on the relationship between free T4 and TSH at 12-16 weeks gestation [most women are recruited after 12 weeks when hCG levels would be expected to have declined]. Furthermore, the low incidence of autoimmunity in women with low free T4 suggests that sub-optimal iodine intake may play a role in thyroid function at this stage of pregnancy.

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