Serum TSH Cutoff in Pregnancy
Pregnancy-Specific TSH Reference Ranges
For pregnant women, use a first-trimester TSH upper limit of 4.0 mIU/L when population-specific reference ranges are not available, or alternatively 0.5 mIU/L below the non-pregnant upper limit. 1
The 2017 American Thyroid Association guidelines revised their previous stricter recommendations after recognizing that the earlier cutoffs (0.1-2.5 mIU/L for first trimester) led to overdiagnosis and unnecessary treatment. 1 The current evidence-based approach acknowledges that:
TSH values vary by gestational week within the first trimester, with non-pregnant reference ranges appropriate up to week 6, and TSH levels approximately 0.4 mIU/L lower than non-pregnancy upper limits during weeks 9-12. 2
The lower limit remains 0.1 mIU/L throughout the first trimester. 2
Treatment Thresholds During Pregnancy
Treat subclinical hypothyroidism in pregnancy when TSH exceeds the trimester-specific upper limit to restore TSH to the reference range. 3
For women with known hypothyroidism on levothyroxine:
Target preconception TSH below 1.2 mIU/L to minimize the need for dose increases during pregnancy. 3, 4 When preconception TSH is 1.2-2.4 mIU/L, 50% of patients require dose increases during pregnancy, compared to only 17.2% when preconception TSH is <1.2 mIU/L. 4
Increase levothyroxine dose by 25-50% immediately upon confirmation of pregnancy in women with pre-existing hypothyroidism, as requirements typically increase substantially during early pregnancy. 5
Clinical Significance of TSH Elevation in Pregnancy
Even mild TSH elevation (2.5-5.0 mIU/L) in thyroid antibody-negative women is associated with increased pregnancy loss. 6 A large prospective study of 4,123 women demonstrated pregnancy loss rates of 6.1% in women with TSH 2.5-5.0 mIU/L versus 3.6% in women with TSH <2.5 mIU/L (p=0.006). 6
Untreated maternal hypothyroidism increases risks of:
Monitoring Protocol During Pregnancy
Check thyroid function tests every 4-6 weeks during the first half of pregnancy and at least once during the second half. 5
For isolated maternal hypothyroxinemia (low T4 with normal TSH), treat with levothyroxine to restore T4 levels to normal range despite normal TSH, as this pattern may adversely affect fetal development. 3
Critical Pitfalls to Avoid
Do not apply uniform TSH limits across the entire first trimester, as this leads to misclassification and unnecessary treatment given the week-by-week variation in early pregnancy. 2
Do not use the outdated 2011 ATA cutoff of 2.5 mIU/L as an absolute treatment threshold unless population-specific data support it, as this was revised in 2017 due to overtreatment concerns. 1
Ensure women with hypothyroidism achieve optimal thyroid function before conception rather than waiting until pregnancy is confirmed, as early fetal neurologic development depends on adequate maternal thyroid hormone. 5, 3