From the Guidelines
The recommended thyroid-stimulating hormone (TSH) levels during pregnancy should be maintained within the reference range to minimize risks of fetal wastage and neuropsychological complications in the offspring, as suggested by the study published in JAMA in 2004 1.
Importance of TSH Levels in Pregnancy
Pregnancy naturally causes changes in thyroid function, including increased thyroid hormone production and decreased TSH levels due to the effects of human chorionic gonadotropin (hCG), which has mild TSH-like activity.
- Maintaining appropriate thyroid function during pregnancy is crucial as maternal thyroid hormones are essential for fetal brain development, especially in the first trimester before the fetal thyroid becomes functional.
- Women with pre-existing hypothyroidism who are on levothyroxine may require increased medication doses during pregnancy, typically by about 30-50%, as indicated in the American Family Physician guidelines from 2013 1.
Monitoring and Adjusting TSH Levels
- TSH levels should be checked every 6-8 weeks during pregnancy, with dose adjustments made to maintain TSH within the reference range, as recommended by the JAMA study in 2004 1.
- Women diagnosed with hypothyroidism during pregnancy should start levothyroxine promptly to normalize TSH levels as quickly as possible, minimizing the risk of adverse outcomes.
Risks of Untreated Hypothyroidism
- Untreated hypothyroidism in pregnancy is associated with increased risks of preterm birth, low birth weight, placental abruption, and fetal death, as highlighted in the American Family Physician guidelines from 2013 1.
- Cognitive impairment in children has also been linked to hypothyroidism in the first trimester, emphasizing the importance of early diagnosis and treatment.
From the FDA Drug Label
In pregnant patients with primary hypothyroidism, maintain serum TSH in the trimester-specific reference range The general aim of therapy is to normalize the serum TSH level
The recommended TSH levels in pregnancy are trimester-specific, and the goal is to normalize the serum TSH level. However, the exact trimester-specific reference ranges for TSH levels are not provided in the drug label. 2
From the Research
TSH Levels in Pregnancy
The recommended thyroid-stimulating hormone (TSH) levels in pregnancy vary depending on the trimester and other factors.
- The American Thyroid Association (ATA) guidelines recommend TSH cutoffs of 0.1 to 2.5 mIU/L in the first trimester, 0.2 to 3.0 mIU/L in the second trimester, and 0.3 to 3.0 mIU/L in the third trimester 3.
- However, other studies have suggested higher cutoffs, with one study recommending an upper cutoff limit of 4.0 mIU/L in the first trimester 4.
- A 2021 study established local pregnancy-related reference intervals for serum TSH in singleton pregnant women, with reference limits of 0.09-3.16 mIU/L in the first trimester, 0.25-3.55 mIU/L in the second trimester, and 0.42-3.93 mIU/L in the third trimester 5.
- Another study found that the first trimester-specific reference intervals for serum TSH were 0.20-4.60 mIU/L in pregnant women, which is different from the ranges suggested by the ATA 6.
- The importance of measuring TSH levels in pregnancy is highlighted by the fact that normal thyroid function is crucial for a healthy pregnancy outcome, and disorders of the thyroid gland are relatively frequent in women of childbearing age 7.
Trimester-Specific Reference Ranges
The following are some trimester-specific reference ranges for TSH levels in pregnancy:
- First trimester: 0.1-2.5 mIU/L (ATA guidelines) 3, 0.09-3.16 mIU/L (2021 study) 5, 0.20-4.60 mIU/L (2019 study) 6
- Second trimester: 0.2-3.0 mIU/L (ATA guidelines) 3, 0.25-3.55 mIU/L (2021 study) 5
- Third trimester: 0.3-3.0 mIU/L (ATA guidelines) 3, 0.42-3.93 mIU/L (2021 study) 5
Clinical Significance
Measuring TSH levels in pregnancy is important for detecting thyroid function abnormalities, which can have significant consequences for the health of the mother and the fetus 7.