Assessment of thyroid function during pregnancy: first-trimester (weeks 9–13) reference intervals derived from Western Australian women

To the Editor: Gilbert and colleagues1 report thyroid function test results in a large number of pregnant women in Western Australia during the first trimester. While assessment of thyroid status is increasingly important in pregnancy, they do not present a strong enough argument for their reference ranges to be adopted. Their controls consisted of only 100 blood donors, and it is not clear whether these were age-matched with patients. Differences between pregnant and non-pregnant thyroid hormone ranges were too small to justify use of separate ranges. We assume from the article that the controls were not screened for thyroid antibodies. Prevalence of thyroid autoimmunity is high in women of reproductive age, whether or not they are pregnant.2