In September 2011, The American Thyroid Association (ATA) published new guidelines on the diagnosis and management of thyroid disease during pregnancy and postpartum. There are many recommendations in the guidelines, but I wanted to highlight one in particular.
"If trimester-specific reference ranges for TSH are not available in the laboratory, the following reference ranges are recommended: First trimester, 0.1-2.5 mIU/L; second trimester, 0.2-3.0 mIU/L; third trimester, 0.3-3.0 mIU/L."
These reference intervals are lower than the non-pregnant reference intervals. This is due to the fact that hCG has mild thyroid-stimulating ability. Therefore, hCG stimulates the thyroid and suppresses TSH. This is most apparent during the first trimester (7-11 weeks) when hCG is at its highest concentration. TSH concentrations actually decrease, although usually not below the normal, non-pregnant, reference interval.
This means that hypothyroidism during pregnancy needs to be defined using these pregnancy-specific reference intervals. Overt hypothyroidism is defined as decreased fT4 with TSH > 2.5 mIU/L. Subclinical hypothyroidism is defined as serum TSH 2.5-10 mIU/L with normal fT4. The ATA recommends treating overt hypothyroidism, but not subclinical hypothyroidism, unless women are also positive for anti-TPO antibodies. When patients are treated, the goal is to achieve the trimester-specific reference intervals listed above.
Interestingly, the ATA recommends that women who are taking T4 therapy have their dose adjusted to achieve a TSH concentration of 2.5 mIU/L before pregnancy. This reduces the risk of hypothyroidism during the first trimester. Likewise once women who are on T4 therapy get pregnant, their T4 therapy should be adjusted to keep them within the pregnancy-specific TSH reference intervals and serum TSH should be monitored approximately every 4 weeks during the first half of pregnancy. Serum TSH should also be checked again between weeks 26 and 32.
Similarly, according to the ATA, euthyroid (normal functioning thyroid gland) women who are not on T4 replacement therapy but are TPO antibody positive, should also have serum TSH monitored every 4 weeks during the first half of pregnancy and again between weeks 26 and 32.