Levothyroxine Dose Adjustments During Pregnancy
Stop Adjusting at 16-20 Weeks Gestation
Levothyroxine dose adjustments should generally be discontinued after 16-20 weeks of gestation, as maternal thyroid hormone requirements plateau by this time and remain stable until delivery. 1
Timing of Dose Increases During Pregnancy
First Half of Pregnancy (Weeks 5-16)
- Levothyroxine requirements increase by approximately 47% during the first half of pregnancy, with the median onset of increase at 8 weeks gestation 1
- The dose increase plateaus by week 16 of gestation and remains stable at this elevated level until delivery 1
- Most levothyroxine dose adjustments occur in the first trimester, with requirements increasing 50% by the end of the first trimester 2
Second and Third Trimesters (After Week 16)
- After the plateau at 16 weeks, the levothyroxine dose typically remains constant through the remainder of pregnancy 1
- Only a small percentage of patients (approximately 10-20%) require additional dose adjustments during the second and third trimesters 3
- Requirements increase to 55% above baseline in the second trimester and 62% in the third trimester, but these represent continuation of the first-trimester increase rather than new adjustments 2
Monitoring Protocol After Week 16
Reduced Monitoring Frequency
- After achieving stable TSH levels by mid-pregnancy (around 16-20 weeks), monitoring can be reduced from every 2-4 weeks to every 4 weeks through the remainder of pregnancy 4, 5
- TSH testing every 4 weeks identifies 92% of abnormal values during pregnancy 5
- The FDA label recommends monitoring TSH every 4 weeks until a stable dose is reached, then at minimum once per trimester 6
Target TSH Levels Throughout Pregnancy
- Maintain TSH <2.5 mIU/L in the first trimester, then within trimester-specific reference ranges for the second and third trimesters 7, 8
- The goal is to maintain free T4 in the high-normal range throughout pregnancy using the lowest possible levothyroxine dose 8
Immediate Postpartum Dose Reduction
Return to Pre-Pregnancy Dose
- Reduce levothyroxine to pre-pregnancy levels immediately after delivery 6
- The increased levothyroxine requirement is specific to pregnancy and resolves with delivery 1
- Monitor serum TSH 4-8 weeks postpartum to confirm appropriate dosing 6
Clinical Rationale for Stopping Adjustments
Physiological Plateau
- The physiological increase in thyroid hormone requirements during pregnancy reaches a plateau by 16 weeks gestation because the placental production of human chorionic gonadotropin (hCG) and estradiol stabilizes at this point 1
- After this plateau, maternal thyroid hormone metabolism remains constant until delivery 1
Risk of Over-Treatment
- Continuing to increase levothyroxine after the plateau phase risks TSH suppression below 0.1 mIU/L, which occurred in 8-15% of pregnant women who received excessive dose increases 5
- TSH suppression during pregnancy may increase risks of maternal complications, though the primary concern is ensuring adequate thyroid hormone for fetal neurodevelopment 5
Common Pitfalls to Avoid
- Do not continue aggressive dose escalation beyond 16-20 weeks gestation, as this leads to unnecessary TSH suppression 5
- Do not wait for TSH results before making the initial dose increase at pregnancy confirmation—increase by 25-30% immediately 1, 8
- Do not forget to reduce the dose immediately postpartum, as failure to do so will result in iatrogenic hyperthyroidism 6
- Do not assume all patients follow the same pattern—approximately 16% of well-controlled hypothyroid women do not require any dose increase during pregnancy 2