Liothyronine (T3) Safety in Pregnancy
Liothyronine (T3) should NOT be used during pregnancy; levothyroxine (T4) monotherapy is the only recommended thyroid hormone replacement for pregnant women with hypothyroidism.
Primary Recommendation
Levothyroxine alone is the standard of care for thyroid hormone replacement during pregnancy, and combination T4+T3 therapy poses theoretical dangers to fetal development. 1 The evidence supporting this recommendation is based on:
- Physiological rationale: The fetus relies entirely on maternal T4 (not T3) during the first trimester for normal neurological development, as the fetal thyroid doesn't produce hormones until mid-gestation 2
- Lack of safety data: There are no controlled studies demonstrating the safety of liothyronine during pregnancy 1
- Professional society consensus: All major guidelines recommend levothyroxine monotherapy, with no endorsement of T3-containing regimens 3, 2, 4
Why Levothyroxine Is Preferred
Levothyroxine is safe during pregnancy and should never be discontinued, as untreated hypothyroidism causes severe adverse maternal and fetal outcomes including preeclampsia, low birth weight, placental abruption, fetal death, and permanent neurodevelopmental deficits. 3, 2
Key advantages of levothyroxine:
- Proven safety profile: Decades of use with well-documented safety in pregnancy 3, 5
- Crosses placenta appropriately: Provides T4 that the fetus converts to T3 as needed 1
- Prevents cognitive impairment: Adequate maternal T4 in the first trimester is essential for normal fetal brain development 2, 6
Management Protocol for Hypothyroid Pregnant Women
Increase levothyroxine dose by 25-30% (approximately 25 µg for a patient on 75 µg) immediately upon pregnancy confirmation, without waiting for TSH results. 2
Monitoring schedule:
- Check TSH and free T4 every 4 weeks during the first half of pregnancy 4
- Check at least once during the second half of pregnancy 4
- Target TSH < 2.5 mIU/L in the first trimester, then maintain within trimester-specific reference ranges 2, 4
Dosing considerations:
- Up to 75% of women require higher levothyroxine doses during pregnancy (mean ~150 µg/day, but highly variable) 7, 6
- Dose increases begin as early as 5-8 weeks gestation and plateau by week 16 6
- After delivery, reduce levothyroxine to pre-pregnancy dose and reassess at 6-12 weeks postpartum 7
Critical Pitfalls to Avoid
Do not wait for TSH results before increasing levothyroxine in a pregnant woman with known hypothyroidism—fetal harm can occur before maternal symptoms appear. 2
Do not use combination T4+T3 therapy during pregnancy, even if the patient was taking it before conception—switch to levothyroxine monotherapy when pregnancy is confirmed or planned. 1
Do not target TSH above 2.5 mIU/L in the first trimester, as even subclinical hypothyroidism is associated with adverse pregnancy outcomes. 2
Preconception Planning
Women with hypothyroidism planning pregnancy should have thyroid function optimized before conception, with a target TSH below 1.2 mIU/L. 4 This prevents the critical early gestational period (when fetal brain development is most vulnerable) from being complicated by maternal hypothyroidism. 2