Synthroid Dose Adjustment During Pregnancy
Pregnant women with pre-existing hypothyroidism should immediately increase their levothyroxine dose by 25-30% as soon as pregnancy is confirmed, without waiting for laboratory confirmation, to prevent irreversible fetal neurocognitive impairment. 1, 2
Immediate Action Upon Pregnancy Confirmation
- Increase the levothyroxine dose by two extra doses per week (approximately 30% increase) immediately when pregnancy is confirmed 2
- Do not delay treatment waiting for TSH results, as levothyroxine requirements increase as early as the fifth week of gestation, and first-trimester hypothyroidism specifically causes permanent cognitive impairment in children 1, 2
- The FDA label supports dose increases of 12.5 to 25 mcg per day for pregnant patients with pre-existing hypothyroidism 3
Target TSH Goals
- Maintain TSH <2.5 mIU/L in the first trimester and within trimester-specific reference ranges thereafter 1, 3
- Keep free T4 in the high-normal range throughout pregnancy 1
- Women with preconception TSH >1.2 mIU/L have a 50% chance of requiring dose increases during pregnancy, compared to only 17% when preconception TSH is <1.2 mIU/L 4
Monitoring Schedule
- Check TSH every 4 weeks during the first half of pregnancy and at minimum once during the second half 5, 3
- The older 2004 JAMA guideline recommended monitoring every 6-8 weeks, but more recent evidence supports more frequent monitoring 6
- Adjust levothyroxine by 12.5-25 mcg increments based on TSH results 1, 3
Expected Dose Increases Throughout Pregnancy
- Most women require 25-50% total dose increase above pre-pregnancy levels 1, 7
- The increase occurs progressively: 13% in first trimester, 26% in second trimester, and 26% in third trimester on average 8
- The magnitude varies by etiology: women with treated Graves' disease or goiter require the largest increases (up to 51% by second trimester), while those with primary hypothyroidism require smaller increases (16% by second/third trimester) 8
- Dose requirements plateau by week 16 of gestation and remain elevated until delivery 2
Postpartum Management
- Reduce levothyroxine dose to pre-pregnancy levels immediately after delivery 3
- Monitor serum TSH 4-8 weeks postpartum 3
Critical Pitfalls to Avoid
- Never wait for symptoms to develop before checking TSH, as fetal harm occurs before maternal symptoms appear 1
- Do not target TSH >2.5 mIU/L in the first trimester, as even subclinical hypothyroidism associates with preeclampsia, preterm birth, low birth weight, placental abruption, and fetal death 1
- Avoid the misconception that adequately treated women don't need dose adjustments—the increased requirement occurs due to increased thyroid binding globulin synthesis and physiologic changes of pregnancy, not inadequate pre-pregnancy treatment 7, 2