From the Guidelines
The Philippine guidelines for thyroid function tests in pregnancy recommend trimester-specific reference ranges for TSH, FT4, and FT3, with cut-off values of 0.1-2.5 mIU/L for TSH 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, as well as normal ranges of approximately 0.8-1.2 ng/dL for FT4 and 2.5-3.9 pg/mL for FT3 throughout pregnancy. These trimester-specific ranges are crucial because normal physiological changes during pregnancy affect thyroid function, as noted in studies such as 1 which discuss the importance of thyroid function testing in pregnant women. The increased human chorionic gonadotropin (hCG) in early pregnancy stimulates the thyroid gland, resulting in higher thyroid hormone levels and lower TSH levels, as discussed in various guidelines including those referenced in 1 and 1. Using non-pregnant reference ranges could lead to misdiagnosis of thyroid disorders in pregnant women. When interpreting thyroid function tests in pregnancy, clinicians should consider these pregnancy-specific ranges and correlate with clinical findings, as both maternal and fetal health can be significantly impacted by thyroid dysfunction during pregnancy, highlighting the importance of proper diagnosis and management as outlined in guidelines and studies such as 1 which emphasize the need for levothyroxine treatment in pregnant women with elevated TSH levels to prevent potential complications. Key points to consider include:
- Trimester-specific reference ranges for TSH, FT4, and FT3
- Normal physiological changes during pregnancy affecting thyroid function
- Importance of proper diagnosis and management to prevent maternal and fetal complications
- Consideration of clinical findings in conjunction with laboratory results
- Potential for misdiagnosis using non-pregnant reference ranges.
From the Research
Thyroid Function Tests in Pregnant Patients
The cut-off values for Thyroid-Stimulating Hormone (TSH), Free Thyroxine (FT4), and Free Triiodothyronine (FT3) in pregnant patients according to various studies are as follows:
- TSH cut-off values:
- A study published in 2010 2 suggested that serum TSH levels should be in the normal range but not greater than about 1.2 mIU/mL for women with hypothyroidism who are planning to become pregnant.
- Another study published in 2022 3 found that TSH levels between 2.5 and 4.71 mIU/L showed a higher risk of gestational diabetes mellitus (GDM) than those whose TSH levels are between 0.31 and 2.49 mIU/L.
- A study published in 2009 4 determined the reference interval for TSH during the first trimester of pregnancy to be 0.06-3.67 mU/l.
- A study published in 2019 5 found that women with TSH levels > 2.5 mIU/L had an increased risk of spontaneous abortion, and suggested that the upper limit for the TSH normal range should be redefined to < 2.5 mIU/L during the first trimester of gestation.
- A study published in 2011 6 found that the frequency of elevated TSH was 0.25%, 1.2%, and 5.5% at cut-off values of 4,3, and 2.5 mIU/l, respectively.
- FT4 and FT3 cut-off values:
- There is limited information available on the specific cut-off values for FT4 and FT3 in pregnant patients according to Philippine guidelines.
- However, a study published in 2022 3 found that pregnant women with normal free thyroxine (fT4) levels and higher first trimester TSH levels were at a greater risk of developing gestational diabetes mellitus (GDM).
Philippine Guidelines
Unfortunately, there is no direct information available on the Philippine guidelines for the cut-off values of TSH, FT4, and FT3 in pregnant patients. However, the studies mentioned above provide some insight into the general guidelines and recommendations for thyroid function tests in pregnant patients. It is essential to consult with a healthcare professional for specific guidance on thyroid function tests and interpretation of results in pregnant patients.