From the Guidelines
Monitoring Hypothyroidism in Pregnancy
The best laboratory test to monitor hypothyroidism in pregnancy is Thyroid-Stimulating Hormone (TSH) levels [ 1 ].
- TSH levels should be monitored regularly, with a target range of 0.1-2.5 mU/L during the first trimester, 0.2-3.0 mU/L during the second trimester, and 0.3-3.5 mU/L during the third trimester.
- Additionally, Free Thyroxine (FT4) levels should also be monitored, with a target range of 0.8-1.2 ng/dL [ 1 ].
- It is recommended to check TSH and FT4 levels every 4-6 weeks during the first trimester and every 6-8 weeks during the second and third trimesters [ 1 ].
- Levothyroxine (T4) is the preferred medication for treating hypothyroidism in pregnancy, with a typical starting dose of 50-100 mcg/day, adjusted as needed to maintain target TSH and FT4 levels [ 1 ].
- The requirement for levothyroxine in treated hypothyroid women frequently increases during pregnancy, so serum TSH concentration should be monitored every 6 to 8 weeks during pregnancy and the levothyroxine dose modified as needed [ 1 ].
From the FDA Drug Label
For pregnant patients with pre-existing hypothyroidism, measure serum TSH and free-T4 as soon as pregnancy is confirmed and, at minimum, during each trimester of pregnancy. In pregnant patients with primary hypothyroidism, maintain serum TSH in the trimester-specific reference range The best laboratory test to monitor hypothyroidism in pregnancy is serum TSH and free-T4, with the goal of maintaining serum TSH in the trimester-specific reference range 2.
- Key laboratory tests:
- Serum TSH
- Free-T4
- Monitoring frequency:
- As soon as pregnancy is confirmed
- At minimum, during each trimester of pregnancy
- Every 4 weeks until a stable dose is reached and serum TSH is within normal trimester-specific range
From the Research
Laboratory Tests for Monitoring Hypothyroidism in Pregnancy
The best laboratory test to monitor hypothyroidism in pregnancy is the measurement of serum thyroid-stimulating hormone (TSH) concentration.
- TSH is considered the most sensitive indicator of thyroid function during pregnancy 3, 4.
- The reference range for TSH varies by trimester, with a recommended upper limit of 2.5 mIU/L in the first trimester, 3.0 mIU/L in the second trimester, and 3.0 mIU/L in the third trimester 3, 4.
- However, some studies suggest that these cutoffs may be too strict, and higher upper limits may be more appropriate 4.
- Other thyroid function tests, such as free thyroxine (FT4) and free triiodothyronine (FT3), may also be useful in certain situations, but TSH is generally considered the primary test for monitoring hypothyroidism in pregnancy 5, 6.
Trimester-Specific Reference Ranges
Trimester-specific reference ranges for TSH are important to ensure accurate diagnosis and treatment of hypothyroidism in pregnancy.
- The reference ranges for TSH vary by trimester, with higher upper limits in the second and third trimesters compared to the first trimester 3, 4.
- The American Thyroid Association (ATA) recommends the following trimester-specific reference ranges for TSH: 0.1-2.5 mIU/L in the first trimester, 0.2-3.0 mIU/L in the second trimester, and 0.3-3.0 mIU/L in the third trimester 4.
- However, other studies suggest that these reference ranges may not be universally applicable, and local population-specific reference ranges may be more appropriate 4, 5.
Clinical Implications
The measurement of TSH is crucial for the diagnosis and treatment of hypothyroidism in pregnancy.
- Overt hypothyroidism is typically defined as a TSH concentration greater than 10 mIU/L, while subclinical hypothyroidism is defined as a TSH concentration between 5 and 10 mIU/L 3.
- Treatment with levothyroxine is generally recommended for women with overt hypothyroidism, while the benefits of treatment for subclinical hypothyroidism are still uncertain 3, 7.
- Regular monitoring of TSH levels is essential to ensure that women with hypothyroidism receive appropriate treatment and to minimize the risk of adverse pregnancy outcomes 7.