From the Guidelines
Monitoring Hypothyroidism in Pregnancy
The best laboratory test to monitor hypothyroidism in pregnancy is Thyroid-Stimulating Hormone (TSH) levels, as it is crucial for assessing the thyroid function and adjusting the levothyroxine dose accordingly 1.
- TSH levels should be monitored every 6 to 8 weeks during pregnancy to ensure that the serum TSH concentration is within the reference range 1.
- Additionally, Free Thyroxine (FT4) levels should also be monitored to assess the effectiveness of levothyroxine therapy 1.
- The target range for TSH levels may vary depending on the trimester, but generally, it is recommended to maintain a TSH level between 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.
- It is essential to adjust the levothyroxine dose as needed to maintain target TSH and FT4 levels, with a typical starting dose of 50-100 mcg/day 1.
- Regular monitoring of TSH and FT4 levels is crucial to prevent potential complications, such as fetal wastage or neuropsychological complications in the offspring due to thyroid insufficiency 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 serum thyroid-stimulating hormone (TSH) concentration, as it is the most sensitive indicator of thyroid function 3, 4, 5, 6, 7.
Interpretation of TSH Values
- TSH values should be interpreted with regard to gestational age, as the reference ranges for TSH vary during different trimesters of pregnancy 5, 6.
- The upper limit of the reference range for TSH in the first trimester is generally considered to be 2.5 mIU/L, while in the second and third trimesters, it is 3 mIU/L 3.
- However, some studies suggest that the upper limit of the reference range for TSH may be higher, up to 4.0 mIU/L, especially in the second and third trimesters 6.
Trimester-Specific Reference Ranges
- The reference ranges for TSH, tri-iodothyronine (T3), and thyroxine (T4) vary during different trimesters of pregnancy 7.
- In the first trimester, the mean T3 and T4 values are lower, while the mean TSH value is higher compared to non-pregnant women 7.
- The reference ranges for T3, T4, and TSH in the second and third trimesters are also different from those in non-pregnant women 7.
Monitoring and Treatment
- Women with overt hypothyroidism should be treated promptly with levothyroxine to achieve serum TSH concentrations within the reference ranges for pregnancy as soon as possible 3, 5.
- Women with subclinical hypothyroidism may also benefit from treatment with levothyroxine, especially if the TSH level is >10 mIU/L or thyroperoxidase antibodies are present 5.
- Close monitoring of serum TSH levels is necessary to avoid overtreatment or undertreatment in pregnant women with hypothyroidism 4, 5.