TSH Goal During Pregnancy
For pregnant women with hypothyroidism, maintain TSH <2.5 mIU/L in the first trimester, <3.0 mIU/L in the second trimester, and <3.0 mIU/L in the third trimester, though the 2017 American Thyroid Association guidelines recommend keeping TSH 0.5 mIU/L below the preconception value or <4.0 mIU/L when population-specific ranges are unavailable. 1
Trimester-Specific TSH Targets
First Trimester (Most Critical Period)
- The optimal TSH target is <2.5 mIU/L, as this is when fetal brain development is most dependent on maternal thyroid hormone 2, 3
- Women with preconception TSH >1.2 mIU/L have a 50% chance of requiring dose increases during pregnancy, compared to only 17.2% when preconception TSH is <1.2 mIU/L 4
- For women planning pregnancy, aim for preconception TSH <1.2 mIU/L to minimize the need for dose adjustments after conception 4
Second and Third Trimesters
- Target TSH <3.0 mIU/L in both second and third trimesters 1, 3
- The FDA label specifies maintaining TSH "in the trimester-specific reference range" without providing exact values 5
Pre-Existing Hypothyroidism Management
Immediate Dose Adjustment Upon Pregnancy Confirmation
- Increase levothyroxine dose by 25-50% immediately upon pregnancy confirmation 6, 5
- This proactive approach prevents the TSH elevation that occurs in most pregnant women with hypothyroidism 4
- Monitor TSH every 4 weeks until stable, then at minimum once per trimester 5, 2
Dose Titration Strategy
- Adjust levothyroxine by 12.5-25 mcg increments based on TSH results 5, 7
- An ongoing adjustment approach (adjusting dose in micrograms per day based on current TSH and levothyroxine dose) is as effective as empiric two-pill-per-week increases 7
- Continue monitoring every 2 weeks in first and second trimesters, then every 4 weeks in third trimester 7
New-Onset Hypothyroidism in Pregnancy
TSH ≥10 mIU/L
- Start levothyroxine at 1.6 mcg/kg/day 5
- Monitor TSH every 4 weeks and adjust until TSH is within trimester-specific range 5
TSH <10 mIU/L
Critical Rationale for Strict TSH Control
Maternal Risks of Inadequate Treatment
- Untreated hypothyroidism increases risk of preeclampsia 8, 6
- Associated with gestational hypertension and stillbirth 6
- Inadequate treatment leads to low birth weight 8, 6
Fetal Neurodevelopmental Risks
- Maternal hypothyroidism from iodine deficiency increases risk of congenital cretinism (mental retardation, growth failure, neuropsychological defects) 8
- Subclinical hypothyroidism and maternal hypothyroxinemia are associated with lower scores on child cognitive assessment 2
- Thyroid hormone is critical for normal fetal brain development, particularly in first and second trimesters 2, 3
Monitoring Protocol
Frequency of Testing
- Measure TSH and free T4 as soon as pregnancy is confirmed 5
- Check TSH every 4 weeks while titrating dose 5, 2
- Once stable, monitor at minimum once per trimester 5
- Some protocols recommend every 2 weeks in first and second trimesters for optimal control 7
Post-Delivery Management
- Reduce levothyroxine to pre-pregnancy dose immediately after delivery 5
- Monitor TSH 4-8 weeks postpartum 5
Common Pitfalls to Avoid
- Never wait for symptoms to develop before checking TSH—fetal harm can occur before maternal symptoms appear 6
- Avoid TSH targets >2.5 mIU/L in first trimester, as even subclinical hypothyroidism is associated with adverse pregnancy outcomes 6, 2
- Do not use combination T3/T4 therapy during pregnancy—levothyroxine monotherapy is the only appropriate treatment as T3 provides inadequate fetal thyroid hormone delivery 6
- Failing to increase the dose proactively upon pregnancy confirmation leads to transient hypothyroidism during the critical first trimester 4
Special Considerations
Women with Positive TPO Antibodies
- Treat even with normal TSH to prevent miscarriage and premature delivery 6
- Target TSH <2.5 mIU/L in first trimester 6