Thyroid Function Testing in Pregnant Women with Gestational Diabetes Mellitus
Pregnant women with GDM should undergo TSH and free T4 (FT4) testing, as thyroid dysfunction occurs more frequently in this population and can significantly impact maternal and fetal outcomes. 1
Recommended Thyroid Function Tests
Order TSH and free T4 (or free thyroxine index/FTI) for all pregnant women with GDM. 1 These are the recommended initial tests for screening and evaluating thyroid disease in pregnancy, as TSH testing using monoclonal antibodies provides the most reliable assessment of thyroid function. 1
Additional Testing Considerations
Anti-thyroid peroxidase antibodies (anti-TPO) should be measured when clinically indicated, particularly if TSH is elevated or thyroid dysfunction is suspected. 1 Research demonstrates that anti-TPO positivity is significantly higher in women with GDM (30.64% vs 8.3% in non-GDM women, p=0.044), and these antibodies correlate with elevated TSH levels. 2
Free T3 (FT3) is not routinely necessary for initial screening but may provide additional information in specific clinical situations. 1 Recent studies show FT3 levels and the FT3/FT4 ratio are significantly elevated in GDM patients compared to controls. 3
Clinical Rationale and Evidence
The association between thyroid dysfunction and GDM is well-established through multiple research studies:
Subclinical hypothyroidism occurs in 38.4% of women with GDM compared to only 14.06% of healthy pregnant women (p=0.016). 4
Mean TSH levels are significantly higher in GDM patients (4.13 vs 2.02, p=0.019). 2
Hypothyroxinemia (low FT4) occurs in approximately 5.6% of high-risk pregnant women. 5
Anti-TPO antibody positivity is present in 18.6-30.64% of women with GDM versus 8.3-10.3% of controls. 2, 4
Timing of Testing
Perform thyroid function testing at the time of GDM diagnosis (typically 24-28 weeks of gestation). 1 For women with additional risk factors (obesity, family history of thyroid disease, prior thyroid dysfunction), consider testing at the first prenatal visit alongside early GDM screening. 6, 7
Clinical Implications
Identifying thyroid dysfunction in women with GDM is critical because:
Untreated maternal hypothyroidism increases the risk of preeclampsia, low birth weight, and impaired infant neurodevelopment. 1, 8
Thyroid hormones affect insulin secretion and beta-cell function, potentially worsening glycemic control. 3
Early treatment of hypothyroidism with levothyroxine can prevent adverse maternal and fetal outcomes. 1
Common Pitfalls to Avoid
Do not rely on HbA1c for thyroid assessment—it has no role in evaluating thyroid function. 1
Do not skip thyroid testing in women with GDM simply because they lack overt symptoms of thyroid disease, as subclinical hypothyroidism is common and asymptomatic. 4
Do not use thyrotropin-releasing hormone (TRH) testing routinely—TSH and FT4 provide adequate assessment in most clinical situations. 1
Recognize that thyroid function changes during pregnancy—use pregnancy-specific reference ranges when interpreting results, as normal TSH ranges are lower in pregnancy. 1
Treatment Goals When Thyroid Dysfunction is Identified
If hypothyroidism is diagnosed, treat with levothyroxine to maintain FT4 or FTI in the high-normal range. 1 Monitor FT4 or FTI every 2-4 weeks during pregnancy to ensure adequate replacement. 1 For hyperthyroidism, use propylthiouracil or methimazole with the goal of maintaining FT4 or FTI in the high-normal range using the lowest possible dose. 1