From the Guidelines
During pregnancy, TSH levels should be maintained below 2.5 mIU/L in the first trimester and below 3.0 mIU/L in the second and third trimesters, while Free T4 should remain within the pregnancy-specific reference range established by each laboratory, typically between 0.8-1.7 ng/dL. These targets are crucial because maternal thyroid hormones are essential for fetal brain development, especially in the first trimester before the fetal thyroid becomes functional [ 1 ]. Pregnancy naturally increases thyroid hormone demands due to increased metabolism, higher blood volume, and the transfer of thyroid hormones to the fetus. Some key points to consider include:
- Women with pre-existing thyroid conditions often need their medication doses adjusted during pregnancy, typically requiring 25-50% higher doses of levothyroxine.
- Regular monitoring every 4-6 weeks during the first half of pregnancy and at least once in the second half is recommended to ensure optimal levels [ 1 ].
- Inadequate thyroid hormone levels during pregnancy can lead to complications including miscarriage, preeclampsia, premature birth, and impaired neurological development in the baby. It is also important to note that the requirement for levothyroxine in treated hypothyroid women frequently increases during pregnancy, and therefore, serum TSH concentration should be monitored every 6 to 8 weeks during pregnancy and the levothyroxine dose modified as needed [ 1 ].
From the Research
Thyroid Function During Pregnancy
The normal levels of free T4 and TSH during pregnancy are crucial for the health of both the mother and the fetus.
- The study 2 found that serum total T4 levels increased into the second trimester and plateaued around 16 weeks, with an upper threshold ranging from 12.6 to 16.4 μg/dL and a lower threshold ranging from 5.3 to 8.0 μg/dL.
- However, the same study 2 also noted that when combined with elevated TSH levels, free or total T4 determinations are equally sensitive to identify women with subclinical hypothyroidism who are at increased risk for preterm birth and placental abruption.
- Unfortunately, the provided studies do not give specific reference ranges for free T4 and TSH during pregnancy.
General Thyroid Function Reference Ranges
Outside of pregnancy, the general reference ranges for thyroid function tests are:
- The study 3 mentioned that the re-test interval for thyroid function tests was much lower than recommended for those with normal TFTs, but it did not provide specific reference ranges for free T4 and TSH.
- The study 4 noted that liquid levothyroxine treatment led to increased free thyroxine (FT4) and decreased TSH concentrations, but it did not provide specific reference ranges for these tests.
Treatment of Hypothyroidism
The treatment of hypothyroidism typically involves levothyroxine replacement therapy, and the goal is to achieve normal levels of TSH and free T4.
- The study 5 noted that diagnosis of hypothyroidism relies on simple-to-obtain testing of thyroid-stimulating hormone (TSH) levels and confirmation with levels of thyroxine.
- The study 6 discussed the importance of getting the levothyroxine dose right for adults with hypothyroidism, but it did not provide specific reference ranges for free T4 and TSH during pregnancy.