Levothyroxine Dose Adjustment in Pregnancy
Increase levothyroxine by 25-30% (approximately two extra doses per week) immediately upon pregnancy confirmation. This proactive approach prevents maternal hypothyroidism during the critical first trimester when fetal brain development depends entirely on maternal thyroid hormone 1, 2.
Timing and Magnitude of Dose Increase
Levothyroxine requirements increase as early as the 5th week of gestation, with the median onset at 8 weeks 1. The increase plateaus by week 16 and remains elevated until delivery 1. Waiting for TSH elevation before adjusting the dose risks fetal neurodevelopmental harm, as maternal hypothyroidism during pregnancy is associated with impaired cognitive development and increased fetal mortality 1.
Evidence-Based Dosing Strategy
- For women with pre-existing hypothyroidism: Increase the pre-pregnancy levothyroxine dose by 25-50% immediately upon pregnancy confirmation 3, 1, 2
- The mean increase required is 47% during the first half of pregnancy, translating to an average additional 36-50 mcg daily 1, 4, 5
- Practical implementation: Add two extra tablets per week of the current dose (approximately 29% increase) as soon as pregnancy is confirmed 2
- Up to 75% of hypothyroid women require higher doses during pregnancy to maintain normal TSH levels 4
Monitoring Protocol During Pregnancy
- Check TSH and free T4 as soon as pregnancy is confirmed, then every 4 weeks until stable, and at minimum once per trimester 3, 2
- Target TSH <2.5 mIU/L in the first trimester and within trimester-specific reference ranges thereafter 3, 6
- Adjust levothyroxine by 12.5-25 mcg increments based on TSH results every 4 weeks 3
- Monitor every 4 weeks through midgestation to identify 92% of abnormal values 2
Critical Preconception Considerations
Women planning pregnancy should have TSH optimized to <1.2 mIU/L before conception 6. When preconception TSH is 1.2-2.4 mIU/L, 50% of patients require dose increases during pregnancy, compared to only 17% when preconception TSH is <1.2 mIU/L 6.
Postpartum Management
- Reduce levothyroxine to pre-pregnancy levels immediately after delivery 3, 4
- Monitor TSH 4-8 weeks postpartum to confirm appropriate dosing 3
- The increased requirement resolves after delivery, and continuing the pregnancy dose risks iatrogenic hyperthyroidism 4
Common Pitfalls to Avoid
- Never wait for TSH elevation before increasing levothyroxine in pregnancy—fetal harm can occur before maternal symptoms appear 7, 1
- Avoid TSH targets >2.5 mIU/L in the first trimester, as even subclinical hypothyroidism is associated with adverse pregnancy outcomes including preeclampsia, low birth weight, and neurodevelopmental effects 7
- Do not use combination T4/T3 therapy during pregnancy—T3 provides inadequate fetal thyroid hormone delivery 7
- Inadequate treatment is associated with low birth weight and potential cognitive impairment in offspring 7
Special Considerations
For women with preconception TSH already <1.2 mIU/L, the risk of requiring dose adjustment is lower (17%), but proactive increase remains recommended given the critical importance of maternal euthyroidism for fetal neurodevelopment 6. Individual variation in levothyroxine requirements during pregnancy is substantial, necessitating individualized monitoring 4, 5.