More Level A Recommendations. There are better data available on several topics, moving more recommendations into level A. In 2002 there was only one Level A recommendation (levels of thyroid-stimulating hormone [TSH] or free thyroxine [measured directly as FT4 or calculated as the free thyroxine index or FTI] should be monitored to manage thyroid disease in pregnancy). In 2015 there are now 7 level A recommendations. The first recommendation is described above (do not do universal screening for thyroid disease in pregnancy). The remaining 6 are paraphrased below and none should be surprising:
1. TSH is the first-line screening test to assess thyroid status in pregnancy.
2. TSH and FT4 should be measured to diagnose thyroid disease in pregnancy.
3. Treat overt hypothyroid disease in pregnancy with adequate thyroid hormone to minimize risk of adverse outcomes.
4. TSH should be monitored in pregnant women who have overt hypothyroidism and the dosage of thyroid replacement adjusted accordingly.
5. Pregnant women with overt hyperthyroidism should be treated with thioamide to minimize risk adverse outcomes.
6. FT4 should be monitored in pregnant women with hyperthyroidism and thioamide dose adjusted accordingly.
TSH levels change in pregnancy. Table 1 in the 2002 Practice Bulletin shows no change in TSH in pregnancy.2 But we know that TSH levels decrease in the first trimester because hCG directly stimulates the TSH receptor, resulting in mild increased T4, which suppresses TSH. Many clinicians use a lower upper limit of normal in the first trimester compared with later in pregnancy. In the first trimester, TSH should be < 2.5 mIU/L compared with < 3.0 mIU/L after the first trimester. 8 Unfortunately, Table 1 in the 2015 Practice Bulletin does not include FTI, which is a very useful measure which can be used instead of free thyroxine, and does not change during pregnancy.
Subclinical hyperthyroidism. A new section has been included on subclinical hyperthyroidism, which is defined as an abnormally suppressed TSH accompanied by a normal FT4 level. Subclinical hyperthyroidism is present in approximately 1.5% of pregnant women.9 No adverse outcomes have been associated with this finding, and so, this is one more reason not to check thyroid function tests routinely.
Thyroid autoantibodies in pregnancy. A new section also has been included on thyroid autoantibodies in pregnancy and whether to screen for them. As you might expect, because data are insufficient to support any benefit of screening and treatment in pregnancy of euthyroid women, universal screening for thyroid autoantibodies in pregnancy is not recommended.