Hypothyroidism During Pregnancy: Different Standard of Care

Pregnancy is a special time for mother and child that requires closer and more involved management of hypothyroidism.

Thyroid hormone is specially important for the fetus, as it is required for proper brain and nervous system development.

Note that the target or normal values for TSH are different during pregnancy.  The American Thyroid Association has released the following recommendations and here are some highlights:

TSH normal ranges are different for every trimester

• 0.1 – 2.5mIU/L for the first trimester
• 0.2 – 3.0mIU/L for the second trimester
• 0.3 – 3.0mIU/L for the third trimester

These lab values are overall lower than the normal ranges for non-pregnant persons. Therefore, if a woman with hypothyroidism is planning to get pregnant, it might be wise to discuss this with her doctor to see if she might need to increase her thyroid medication dose to get her TSH levels lower in preparation for pregnancy.

A common question at this time is the usefulness of measuring T4. While it is still useful to be measure, be prepared to know that T4 levels using the most common type of test (called the immunoassay test) is less accurate during pregnancy. This is because there are now other molecules/proteins, such as thyroid binding globulin, in a pregnant woman’s blood that alter the accuracy of the test. These molecules/proteins include increasing levels of thyroid binding globulin (TBG), increases in non-esterified fatty acid levels and decreasing levels of albumin. Newer tests such as liquid-chromatography-tandem mass spectrometry (LC/MS/MS) testing is seen to be more accurate than the more commonly available immunoassay test, and it is a good option for more accurately measuring T4 if this test is available in a community or laboratory.

The testing schedule and follow-up is more involved

In a non-pregnant person, follow-up for thyroid labs and adjustments to thyroid medication may happen only twice to once a year, especially if the hypothyroidism seems steady and “well-managed.” However, during pregnancy, the follow-ups tend to be more frequent–partly because of the many and rapid changes that happen day-to-day/week-to-week/month-to-month during pregnancy, and partly because the importance of thyroid hormone to the developing fetus is much higher.

A rough outline of a recommended schedule includes:

• Lab assessment of TSH and T4 levels every 4 weeks during the first half of pregnancy to ensure that target levels are reached
• Regular reassessment of TSH and T4 levels during the second half of pregnancy, the frequency decided between the patient and doctor depending on how stable the pregnancy, labs, signs & symptoms are

Finally, it is now also recommend that for women who are already taking thyroid hormone, that extra doses of thyroid hormone be taken once the pregnancy is confirmed.  The recommendation is two extra doses per week (one dose taken several days apart x 2) of the current dosage level may be started once a pregnancy is confirmed.