Management of Hypothyroidism in Pregnancy
Pregnant women with hypothyroidism require immediate levothyroxine treatment with aggressive dose titration to maintain TSH <2.5 mIU/L in the first trimester, as untreated maternal hypothyroidism significantly increases risks of miscarriage, preeclampsia, preterm birth, and permanent neurocognitive impairment in offspring. 1
Pre-Pregnancy Optimization
- Women planning pregnancy should achieve TSH <1.2 mIU/L before conception, as 50% of women with pre-conception TSH between 1.2-2.4 mIU/L require dose increases during pregnancy, compared to only 17.2% when TSH is <1.2 mIU/L 2
- Educate patients that levothyroxine requirements will increase by 25-50% during pregnancy 1
- Women adequately treated before pregnancy have no increased risk of perinatal morbidity 1
Immediate Action Upon Pregnancy Confirmation
- Increase levothyroxine dose by 30-50% immediately upon positive pregnancy test without waiting for laboratory confirmation, as first-trimester hypothyroidism specifically causes cognitive impairment in children 1, 3
- Do not delay treatment waiting for repeat testing 1
- For women already on levothyroxine, an empirical dose increase of at least two additional doses per week (or daily increase of 30-50%) should be implemented immediately 3
Monitoring Protocol During Pregnancy
- Check TSH every 4 weeks during the first half of pregnancy, then at least once between 26-32 weeks 1
- Target TSH <2.5 mIU/L in the first trimester, then use trimester-specific reference ranges for second and third trimesters 1, 3
- Maintain free T4 in the high-normal range throughout pregnancy 1
- Adjust levothyroxine by 12.5-25 mcg increments based on TSH results 1
Dosing Strategies by Clinical Scenario
For women inadequately treated but known to be hypothyroid:
- Double the levothyroxine dose on at least three days per week to rapidly achieve euthyroidism 3
For newly diagnosed overt hypothyroidism in pregnancy:
- Starting doses of 100-150 mcg daily are safe and appropriate 3
For women on stable pre-pregnancy doses:
- Most require 25-50% dose increase above pre-pregnancy levels 1
Postpartum Management
- Return to pre-pregnancy levothyroxine dose immediately after delivery, as postpartum TSH levels return to preconception values 4
- Levothyroxine is safe during breastfeeding and may normalize milk production in hypothyroid lactating mothers 4
Critical Safety Considerations
- Untreated or inadequately treated maternal hypothyroidism increases risks of: miscarriage, gestational hypertension, preeclampsia, preterm birth (22-44% incidence), low birth weight, placental abruption, stillbirth, and fetal death 1, 5
- Fetal neurocognitive development is permanently impaired by first-trimester hypothyroidism, making early aggressive treatment essential 1, 4
- Levothyroxine should never be discontinued during pregnancy 4
Common Pitfalls to Avoid
- Avoid overtreatment: TSH <0.10 mIU/L during pregnancy increases odds of preterm delivery (adjusted OR 2.14) 6
- Do not target TSH >2.5 mIU/L in the first trimester, as even subclinical hypothyroidism associates with adverse pregnancy outcomes 1
- Do not wait for symptoms to develop before checking TSH, as fetal harm occurs before maternal symptoms appear 1
- Approximately 18% of women on thyroid replacement before pregnancy receive no TSH testing during pregnancy—this represents a critical gap in care 6