Levothyroxine Use in Pregnancy
Yes, pregnant individuals with hypothyroidism should absolutely take levothyroxine—it is essential for both maternal health and fetal neurodevelopment, and discontinuing it poses serious risks. 1, 2
Why Levothyroxine is Critical During Pregnancy
Untreated maternal hypothyroidism during pregnancy is associated with severe complications including spontaneous abortion, gestational hypertension, pre-eclampsia, stillbirth, premature delivery, and impaired fetal neurocognitive development. 2 The risks of not treating far outweigh any concerns about medication use during pregnancy. 1, 3
Levothyroxine is present in human milk and no adverse effects on breastfed infants have been reported, making it safe throughout pregnancy and lactation. 2
Dose Adjustments Required During Pregnancy
Pregnancy increases levothyroxine requirements in 84% of women with well-controlled hypothyroidism, with increases needed as early as the fifth week of gestation. 4, 5 This is not optional—failure to increase the dose results in maternal hypothyroidism that harms the developing fetus.
Immediate Action Upon Pregnancy Confirmation
Women already taking levothyroxine should increase their dose by 30-50% immediately upon pregnancy confirmation. 5, 6 This empirical increase prevents the biochemical hypothyroidism that occurs in many pregnant women when doses are not adjusted proactively. 5
The median onset of increased levothyroxine requirement is 8 weeks of gestation, with requirements increasing 47% during the first half of pregnancy and plateauing by week 16. 5 Waiting for TSH elevation before adjusting doses risks fetal harm during critical early neurodevelopment. 5
Magnitude of Dose Increases
Levothyroxine requirements increase progressively throughout pregnancy: 4
- 50% increase in the first trimester 4
- 55% increase in the second trimester 4
- 62% increase in the third trimester 4
These increased doses are required until delivery. 5
Monitoring Protocol
Check TSH every 4 weeks during dose titration, then every trimester once stable. 1, 7 The goal is maintaining TSH below 2.5 mIU/L, ideally within trimester-specific reference ranges, with Free T4 in the upper half of the normal range. 1, 6
For women with known hypothyroidism but inadequate treatment, doubling the levothyroxine dose on at least three days per week rapidly achieves euthyroidism. 6
Postpartum Management
Immediately after delivery, return to the pre-pregnancy levothyroxine dose. 2 Postpartum TSH levels are similar to preconception values, making the pregnancy dose excessive once the pregnancy-related increased requirements resolve. 2
Critical Safety Considerations
Levothyroxine should never be discontinued during pregnancy. 2 The FDA label explicitly states this, and clinical experience confirms no increased rates of major birth defects, miscarriages, or adverse maternal/fetal outcomes with levothyroxine treatment. 2
Do not use combination levothyroxine + liothyronine (T3) therapy during pregnancy. 8 T3 supplementation provides inadequate fetal thyroid hormone delivery because the fetal brain requires T4, not T3, for proper neurodevelopment. 8 Levothyroxine monotherapy is the only appropriate treatment. 1
Common Pitfalls to Avoid
- Waiting for TSH elevation before increasing the dose: Increase empirically by 30-50% as soon as pregnancy is confirmed. 5, 6
- Using pre-pregnancy TSH targets: Target TSH <2.5 mIU/L in pregnancy, not the standard 0.5-4.5 mIU/L range. 6
- Forgetting to reduce the dose postpartum: Return to pre-pregnancy dose immediately after delivery to avoid iatrogenic hyperthyroidism. 2
- Inadequate monitoring frequency: Check TSH every 4 weeks during titration, not every 6-8 weeks as in non-pregnant patients. 1