Levothyroxine Management During Pregnancy
Pregnant women with hypothyroidism should increase their levothyroxine dose by approximately 30% as soon as pregnancy is confirmed, with subsequent dose adjustments based on thyroid function monitoring every 4 weeks throughout pregnancy. 1
Dosing Guidelines for Hypothyroidism in Pregnancy
Pre-existing Hypothyroidism
- For women with pre-existing hypothyroidism, measure serum TSH and free-T4 as soon as pregnancy is confirmed and at minimum during each trimester 2
- Increase levothyroxine dosage by 12.5 to 25 mcg per day when pregnancy is confirmed 2
- On average, women require a 47% increase in levothyroxine dose during the first half of pregnancy, with the need for increased dosage beginning as early as the fifth week of gestation 1
- The magnitude of required increase varies based on the etiology of hypothyroidism:
Monitoring and Dose Adjustments
- Monitor TSH every 4 weeks until a stable dose is reached and serum TSH is within normal trimester-specific range 2
- Maintain serum TSH in the trimester-specific reference range throughout pregnancy 2
- Return to pre-pregnancy levothyroxine dosage immediately after delivery 2
- Monitor serum TSH 4-8 weeks postpartum 2
Special Considerations
Subclinical Hypothyroidism
- For subclinical hypothyroidism (elevated TSH with normal T4), treatment with levothyroxine is recommended during pregnancy 4
- Women with subclinical hypothyroidism who wish to become pregnant should be treated with levothyroxine to restore serum TSH to the reference range 4
- The benefit-risk ratio of levothyroxine therapy in pregnancy justifies its use even without published intervention trials specifically for this population 4
Low T4 with Normal TSH
- Pregnant women with low T4 and normal TSH during pregnancy should be treated with levothyroxine to restore T4 levels to the normal range, despite normal TSH values 5
- This pattern may represent isolated maternal hypothyroxinemia, which can have adverse effects on fetal development 5
- Adjust levothyroxine dose to maintain Free T4 in the upper normal range 5
Preconception Planning
- For women planning pregnancy, aim for preconception TSH below 1.2 mIU/L, as only 17.2% of women with preconception TSH <1.2 mIU/L required dose increases during pregnancy, compared to 50% of those with TSH 1.2-2.4 mIU/L 6
- Women with hypothyroidism should be counseled about the importance of achieving optimal thyroid function before conception 4
Risks of Untreated Hypothyroidism in Pregnancy
- Untreated maternal hypothyroidism increases the risk of preeclampsia 4
- Inadequate treatment is associated with low birth weight in neonates 4
- Maternal hypothyroidism from iodine deficiency increases the risk of congenital cretinism (growth failure, mental retardation, neuropsychologic defects) 4
- Maternal hypothyroxinemia has been associated with impaired fetal neuropsychological development 5
Practical Dosing Approach
- For subclinical hypothyroidism with baseline TSH 2.5-5.0 mIU/L: start with 50 μg/day 7
- For subclinical hypothyroidism with baseline TSH 5.0-8.0 mIU/L: start with 75 μg/day 7
- For subclinical hypothyroidism with baseline TSH >8.0 mIU/L: start with 100 μg/day 7
- These dosages maintain appropriate TSH levels in 79-90% of women, though 10-21% may require additional adjustments during pregnancy 7
Common Pitfalls to Avoid
- Failing to increase levothyroxine dose early in pregnancy can lead to maternal hypothyroidism and potential fetal complications 1
- Waiting until thyroid function tests show abnormalities before adjusting the dose may result in a period of inadequate treatment 1
- Not monitoring thyroid function frequently enough during pregnancy (recommended every 4 weeks) 2, 1
- Failing to reduce the dose back to pre-pregnancy levels after delivery 2