Selecting screening tests. Selecting only one unique test for thyroid dysfunction screening in pregnancy has been claimed to be inadequate because serum TSH and T4 levels may not always be coincident. Preliminary data of The Controlled Antenatal Thyroid Screening (CATS) study showed that half of so-called hypothyroid pregnant women displayed low T4 serum levels without a high TSH, and this may or may not be apathological state.41 Anequal number of women have high TSH serum concentrations,with little change inserumT4 (so-called mild thyroid failure). In fact, very few women have both a low T4 and a high TSH.41 To avoid misinterpretation, we should bearin mind that low TSH serum levels can also be a physiological response during the first trimester of pregnancy. Whereas agreement exists on the usefulness of TSH measurements, different opinions persist on whether serum total T4 (TT4) or FT 4 determination is the complementary test of choice (Table 2). We normally use a T4 index with an appropriate binding assessment, such as TBG or a T3 resinbinding assay, as such indices give results in pregnancy similar to those in normal women. In view of the association between anti-TPO and pregnancy loss and the usefulness of anti-TPO in predicting postpartum thyroiditis, it would also be appropriate to consider anti-TPO in any thyroid screening.42,43
Neck ultrasound. Although scintigraphy scans are contraindicated during pregnancy and rarely needed these days anyway, routine ultrasound may be considered when a goiter or nodular disease is suggested by clinical history and examination. This procedure is useful both to characterize thyroid size and degree of thyroiditis present and to delineate all nodules, allowing an evaluation of their growth characteristics on repeated measurements. In addition, sonography can help make the clinical diagnosis of Graves’ disease (GD) (by excluding nodules) or Hashimoto’s thyroiditis (by the typical
heterogeneous patterning). Of course, the presence of a thyroid nodule does not exclude the diagnosis of GD and would normally initiate radioiodine uptake studies in a nonpregnant patient.