From the Guidelines
The best laboratory test to monitor a patient with hyperthyroidism during pregnancy is Free Thyroxine (FT4) and Thyroid-Stimulating Hormone (TSH) levels. According to the American College of Obstetricians and Gynecologists (ACOG) practice bulletin on thyroid disease in pregnancy 1, TSH and FT4 or FTI testing should be performed in pregnant women with suspected hyperthyroidism or hypothyroidism. The goal is to maintain FT4 or FTI in the high-normal range using the lowest possible thioamide dosage, with measuring the FT4 or FTI every two to four weeks being helpful 1.
- Key laboratory tests include:
- TSH testing
- FT4 or FTI testing
- Medications used to manage hyperthyroidism in pregnancy include:
- Propylthiouracil (PTU)
- Methimazole
- Regular monitoring of liver function tests and complete blood counts is also recommended for patients taking these medications 1.
From the FDA Drug Label
Thyroid function tests should be monitored periodically during therapy Because methimazole crosses placental membranes and can induce goiter and cretinism in the developing fetus, hyperthyroidism should be closely monitored in pregnant women and treatment adjusted such that a sufficient, but not excessive, dose be given during pregnancy In many pregnant women, the thyroid dysfunction diminishes as the pregnancy proceeds; consequently, a reduction of dosage may be possible.
The best laboratory test to monitor a patient with hyperthyroidism during pregnancy is thyroid function tests. These tests should be monitored periodically to ensure that the treatment is effective and to adjust the dose as needed 2.
From the Research
Laboratory Tests for Hyperthyroidism in Pregnancy
To monitor a patient with hyperthyroidism during pregnancy, the following laboratory tests are considered:
- Thyroid-stimulating hormone (TSH) levels: TSH is the primary test for thyroid screening in pregnancy 3
- Free thyroxine (FT4) levels: FT4 is an important indicator of thyroid function, and its levels decrease gradually with increasing gestational age 4
- Total thyroxine (T4) levels: T4 levels increase substantially during pregnancy due to increased thyroxine-binding globulin (TBG) concentrations 4
- Total triiodothyronine (T3) levels: T3 levels can be affected by pregnancy, and reference intervals may vary depending on the trimester and population 5
Reference Intervals for Thyroid Function Tests
Trimester-specific reference intervals are essential for accurate interpretation of thyroid function test results in pregnant women:
- First-trimester reference intervals: 0.16 to 2.82 mIU/L for TSH, 12.0 to 18.5 pmol/L for FT4, 62.8 to 177.9 nmol/L for T4, and 1.5 to 3.4 nmol/L for T3 5
- Second-trimester reference intervals: 0.40 to 3.62 mIU/L for TSH, 10.2 to 16.6 pmol/L for FT4, 66.6 to 176.0 nmol/L for T4, and 1.56 to 3.6 nmol/L for T3 5
- The use of multiples of the median (MoMs) can help overcome limitations of conventional units and inter-laboratory differences 6
Considerations for Laboratory Assessment
When assessing thyroid function in pregnant women, it is crucial to consider the following factors: