What levothyroxine dose adjustment is recommended for a pregnant woman with pre‑existing hypothyroidism who is currently taking 75 µg daily?

Levothyroxine Dose Adjustment in Pregnancy for Pre-existing Hypothyroidism

For a pregnant woman with pre-existing hypothyroidism currently taking 75 µg levothyroxine daily, increase the dose immediately by 25–30% (to approximately 100 µg daily) as soon as pregnancy is confirmed, then monitor TSH every 4 weeks to titrate further as needed. 1, 2, 3

Immediate Dose Increase Upon Pregnancy Confirmation

• Increase levothyroxine by 25–50% (typically 25–30% for most patients) as soon as pregnancy is confirmed, without waiting for TSH results 1, 2, 4, 3
• For a patient on 75 µg daily, this translates to an increase of approximately 20–25 µg, bringing the dose to 95–100 µg daily 3, 5
• Levothyroxine requirements increase as early as the fifth week of gestation, with a median onset at eight weeks, making proactive dose adjustment critical 3
• The mean levothyroxine requirement increases by 47–50% during the first half of pregnancy and plateaus by week 16 3, 5

Rationale for Immediate Empirical Dose Increase

• Untreated or inadequately treated maternal hypothyroidism increases risk of preeclampsia, low birth weight, placental abruption, fetal death, and permanent neurodevelopmental deficits in the child 1, 6, 7, 4
• Hypothyroidism in the first trimester is specifically associated with cognitive impairment in children 1
• Maternal euthyroidism is essential for normal fetal cognitive development, particularly in the first and second trimesters when the fetus is entirely dependent on maternal thyroid hormone 1, 7
• Women who are adequately treated before pregnancy and those diagnosed and treated early in pregnancy have no increased risk of perinatal morbidity 1

Monitoring Protocol During Pregnancy

• Measure TSH and free T4 every 4 weeks after the initial dose increase until TSH is stable within the trimester-specific reference range 2, 4, 8
• Target TSH should be maintained below 2.5 mIU/L in the first trimester, then within trimester-specific reference ranges thereafter 1, 2, 4, 9
• The goal is to maintain free T4 in the high-normal range using the lowest possible medication dose 7
• After each dose adjustment, recheck TSH and free T4 in 4 weeks 2, 4

Subsequent Dose Titration Strategy

• If TSH remains above the trimester-specific reference range at the first prenatal visit, increase levothyroxine by an additional 12.5–25 µg per day 2, 4
• For women who are known to be hypothyroid but inadequately treated, consider doubling the levothyroxine dose on at least three days per week to rapidly achieve euthyroidism 4
• Continue monitoring TSH every 4 weeks and adjust levothyroxine dose by 12.5–25 µg increments based on TSH results 7, 2
• Most levothyroxine dose adjustments are made in the first trimester of gestation 5

Magnitude of Dose Increase Throughout Pregnancy

• Levothyroxine requirements typically increase by 50% in the first trimester, 55% in the second trimester, and 62% in the third trimester 5
• In 84% of well-controlled hypothyroid pregnancies, levothyroxine dosage needs to be increased 5
• The increased dose is required until delivery 3

Preconception TSH Target

• Women planning pregnancy should have TSH optimized to below 2.5 mIU/L—ideally below 1.2 mIU/L—before conception 4, 9
• When preconception TSH is 1.2–2.4 mIU/L, 50% of patients require a dose increase during pregnancy 9
• When preconception TSH is below 1.2 mIU/L, only 17.2% require a dose increase during pregnancy 9

Postpartum Management

• Reduce levothyroxine dose to pre-pregnancy levels immediately after delivery 2
• Monitor serum TSH 4–8 weeks postpartum to confirm appropriate dosing 2

Critical Pitfalls to Avoid

• Never wait for TSH results before increasing levothyroxine in a pregnant woman with known hypothyroidism—fetal harm can occur before maternal symptoms appear 1, 7, 3
• Avoid TSH targets above 2.5 mIU/L in the first trimester, as even subclinical hypothyroidism is associated with adverse pregnancy outcomes 1, 7, 9
• Levothyroxine overtreatment (TSH below 0.10 mIU/L) during pregnancy is associated with preterm delivery, so avoid excessive dose increases 8
• Do not discontinue levothyroxine during pregnancy, as untreated maternal hypothyroidism increases risk of preeclampsia, gestational hypertension, stillbirth, and premature delivery 7
• Approximately 17.8% of women on thyroid replacement before conception do not have TSH measured during pregnancy, representing a critical gap in care 8

Special Considerations

• Levothyroxine monotherapy is the only appropriate treatment during pregnancy, as T3 supplementation provides inadequate fetal thyroid hormone delivery 7
• Instruct patients to take levothyroxine on an empty stomach, 30–60 minutes before breakfast, for optimal absorption 7
• In newly diagnosed overt hypothyroidism during pregnancy (TSH ≥10 mIU/L), starting doses of 100–150 µg daily may be considered safe 4
• For new-onset hypothyroidism with TSH below 10 mIU/L, start at 1.0 mcg/kg/day 2