What is the recommended management for a pregnant woman with hypothyroidism?

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 or inadequately treated maternal hypothyroidism significantly increases risks of miscarriage, preeclampsia, preterm birth, low birth weight, placental abruption, fetal death, and permanent neurocognitive impairment in offspring. 1, 2, 3

Pre-Pregnancy Optimization

For women with known hypothyroidism planning pregnancy:

• Target TSH <1.2 mIU/L before conception to minimize the need for dose adjustments during pregnancy, as 50% of women with pre-conception TSH 1.2-2.4 mIU/L require dose increases during pregnancy compared to only 17.2% with TSH <1.2 mIU/L 4
• Confirm adequate treatment before attempting conception, as women adequately treated before pregnancy have no increased risk of perinatal morbidity 1
• Educate patients that levothyroxine requirements will increase by 25-50% during pregnancy 1, 2, 3

Immediate Management Upon Pregnancy Confirmation

For women already on levothyroxine:

• Increase levothyroxine dose by 30-50% immediately upon positive pregnancy test without waiting for laboratory confirmation 1, 3, 5
• Alternatively, double the levothyroxine dose on at least three days per week to rapidly achieve euthyroidism 3
• Check TSH and free T4 within 4 weeks of pregnancy confirmation 2, 5

For newly diagnosed overt hypothyroidism in pregnancy:

• Start levothyroxine 100-150 mcg daily immediately to rapidly normalize thyroid function 3
• Do not delay treatment waiting for repeat testing, as first-trimester hypothyroidism is associated with cognitive impairment in children 1

Monitoring Protocol During Pregnancy

TSH monitoring frequency:

• Check TSH and free T4 every 4 weeks during the first half of pregnancy while titrating doses 2, 5
• Once stable, check at minimum once per trimester 5
• Target TSH <2.5 mIU/L in the first trimester, then use trimester-specific reference ranges 3, 5

Dose adjustments:

• Adjust levothyroxine by 12.5-25 mcg increments based on TSH results 1
• Most women require 25-50% dose increase above pre-pregnancy levels 1, 2, 3
• Maintain free T4 in the high-normal range throughout pregnancy 1

Critical Safety Considerations

Risks of inadequate treatment:

• Untreated hypothyroidism increases risk of spontaneous abortion, gestational hypertension, preeclampsia, stillbirth, and premature delivery 2
• Maternal hypothyroidism has adverse effects on fetal neurocognitive development 2, 6
• First-trimester hypothyroidism specifically causes cognitive impairment in children 1
• Clinical and subclinical hypothyroidism increase risk of preterm birth, low birth weight, placental abruption, and fetal death 1

Risks of overtreatment:

• TSH <0.10 mIU/L during pregnancy increases odds of preterm delivery 2.14-fold 7
• Avoid excessive levothyroxine doses that suppress TSH below 0.10 mIU/L 7

Postpartum Management

Immediate dose adjustment after delivery:

• Return to pre-pregnancy levothyroxine dose immediately after delivery, as postpartum TSH levels return to pre-conception values 2
• Check TSH 6-8 weeks postpartum to confirm appropriate dosing 2

Common Pitfalls to Avoid

• Never discontinue levothyroxine during pregnancy, even if TSH normalizes, as requirements fluctuate throughout gestation 2, 5
• Do not wait for symptoms to develop before checking TSH, as fetal harm occurs before maternal symptoms appear 1
• Avoid targeting TSH >2.5 mIU/L in the first trimester, as even subclinical hypothyroidism associates with adverse pregnancy outcomes 1, 5
• Do not delay treatment initiation in newly diagnosed cases—start immediately at full replacement doses of 100-150 mcg daily 3
• Approximately 18% of pregnant women on thyroid replacement do not receive TSH testing during pregnancy, representing a critical gap in care 7

Special Considerations

Levothyroxine is safe during pregnancy and lactation:

• Levothyroxine is present in breast milk but causes no adverse effects on breastfed infants 2
• Adequate levothyroxine treatment may normalize milk production in hypothyroid lactating mothers 2
• Clinical experience shows no increased rates of major birth defects, miscarriages, or adverse maternal/fetal outcomes with levothyroxine treatment 2

Women with thyroid autoimmunity:

• Those with positive TPO antibodies and TSH above trimester-specific reference ranges should receive levothyroxine 5
• TPO antibody-positive women have double the risk of miscarriage and postpartum thyroiditis 6