Target TSH in Pregnancy
For pregnant women with hypothyroidism, target TSH <2.5 mIU/L in the first trimester, <3.0 mIU/L in the second and third trimesters. 1
Trimester-Specific TSH Targets
The target TSH ranges differ significantly from non-pregnant populations and vary by trimester 1:
- First trimester: TSH 0.1-2.5 mIU/L 2
- Second trimester: TSH 0.2-3.0 mIU/L 2
- Third trimester: TSH 0.3-3.5 mIU/L 2
These trimester-specific ranges are critical because inadequate treatment directly causes preeclampsia, low birth weight, and potential neurodevelopmental effects in offspring 1.
Pre-Conception Optimization
Women with known hypothyroidism planning pregnancy should achieve TSH <1.2 mIU/L before conception, not just <2.5 mIU/L. 3 This lower target is essential because:
- When pre-conception TSH is 1.2-2.4 mIU/L, 50% of women require levothyroxine dose increases during pregnancy 3
- When pre-conception TSH is <1.2 mIU/L, only 17.2% require dose increases 3
- Women with Hashimoto thyroiditis need even stricter control, with optimal pre-conception TSH cutoffs of 1.24 mIU/L for those already on levothyroxine and 1.73 mIU/L for euthyroid women to prevent first-trimester TSH >2.5 mIU/L 4
Immediate Management Upon Pregnancy Confirmation
Increase levothyroxine dose by 30-50% (or double the dose on at least three days per week) immediately upon pregnancy confirmation in women with pre-existing hypothyroidism 1. This proactive approach is critical because:
- Waiting for TSH elevation wastes critical weeks of fetal neurodevelopment 1
- Levothyroxine requirements increase during pregnancy in most women with hypothyroidism 5, 3
- The FDA recommends monitoring TSH every 4 weeks until stable, then at minimum during each trimester 5
Monitoring Frequency
Monitor TSH every 4 weeks during the first trimester after any dose adjustment, then at minimum once per trimester 1, 5. This differs from the standard 6-8 week intervals used in non-pregnant patients 1.
Treatment Thresholds
For newly diagnosed hypothyroidism in pregnancy 5:
- TSH ≥10 mIU/L: Start levothyroxine 1.6 mcg/kg/day, monitor TSH every 4 weeks 5
- TSH <10 mIU/L but elevated: Start levothyroxine 1.0 mcg/kg/day, monitor TSH every 4 weeks 5
Clinical Significance of TSH Elevation
TSH >2.5 mIU/L in the first trimester increases spontaneous abortion risk 3.7-fold compared to TSH 0.1-2.5 mIU/L 6. TSH >4.6 mIU/L increases this risk nearly 6-fold 6. Treatment with levothyroxine reduces miscarriage rates from 28.6% to 9.8% in women with elevated TSH 6.
Critical Pitfalls to Avoid
- Never use non-pregnant TSH reference ranges (0.45-4.5 mIU/L) during pregnancy, as this leads to undertreatment 1, 2
- Never wait for symptoms before checking TSH, as fetal harm occurs before maternal symptoms appear 1
- Never delay dose adjustment until the next prenatal visit if TSH is elevated—adjust immediately 1
- Ensure levothyroxine is taken on an empty stomach, 30-60 minutes before food, and separated from prenatal vitamins by at least 4 hours, as improper timing reduces absorption by 40-50% 1
Post-Partum Management
Reduce levothyroxine to pre-pregnancy dose immediately after delivery and monitor TSH 4-8 weeks postpartum 5.