What is the recommended management of hypothyroidism medication, specifically levothyroxine (T4), during pregnancy to maintain normal thyroid-stimulating hormone (TSH) levels?

Hypothyroid Medication Management in Pregnancy

Immediate Dose Adjustment Upon Pregnancy Confirmation

Women with pre-existing hypothyroidism should increase their levothyroxine dose by approximately 30% (or add two extra pills per week) as soon as pregnancy is confirmed, typically by 4-5 weeks gestation. 1, 2

• Levothyroxine requirements increase as early as the fifth week of gestation, with the median onset at 8 weeks 1
• The mean increase needed is 47% above pre-pregnancy dose, plateauing by week 16 of gestation 1
• Up to 75% of women with hypothyroidism require higher doses during pregnancy to maintain normal TSH levels 3
• This increased dose must be maintained until delivery 1

TSH Monitoring Schedule During Pregnancy

Monitor serum TSH and free T4 every 4 weeks throughout the first and second trimesters, then every 4 weeks in the third trimester, adjusting levothyroxine to maintain TSH within trimester-specific reference ranges. 4, 5

• Check TSH as soon as pregnancy is confirmed 4
• Continue monitoring at minimum during each trimester, though every 4 weeks is preferred for optimal control 4, 5
• TSH elevations can appear as early as 4-8 weeks gestation or as late as the third trimester 3

Target TSH Ranges in Pregnancy

Maintain serum TSH in the trimester-specific reference range, generally <2.5 mIU/L in the first trimester and <3.0 mIU/L in the second and third trimesters. 2

• For women with pre-existing hypothyroidism, maintain TSH at preconception values throughout pregnancy 1
• Free T4 should be maintained in the high-normal range using the lowest possible medication dose 6
• The upper half of the normal range for free T4 is the target 6

Dose Adjustment Algorithm During Pregnancy

For new TSH elevations during pregnancy, increase levothyroxine by 12.5-25 mcg per day based on the degree of TSH elevation. 4

For Pre-Existing Hypothyroidism with TSH Above Normal Range:

• Increase by 12.5-25 mcg per day 4
• Recheck TSH every 4 weeks until stable and within normal trimester-specific range 4

For New-Onset Hypothyroidism During Pregnancy:

• If TSH ≥10 mIU/L: Start levothyroxine at 1.6 mcg/kg/day 4
• If TSH <10 mIU/L: Start at 1.0 mcg/kg/day 4
• Monitor TSH every 4 weeks and adjust until within normal trimester-specific range 4

Special Considerations for Maternal Hypothyroxinemia

Pregnant women with low free T4 but normal TSH (isolated maternal hypothyroxinemia) should be treated with levothyroxine to restore T4 levels to the normal range, despite normal TSH values. 7

• This condition has been associated with impaired fetal neuropsychological development and increased risk of fetal loss 6, 7
• Monitor thyroid function every 4 weeks during pregnancy 7
• Adjust levothyroxine to maintain free T4 in the upper normal range 7

Postpartum Management

Reduce levothyroxine dose to pre-pregnancy levels immediately after delivery. 4

• Monitor serum TSH 4-8 weeks postpartum 4
• Reassess the need for continued therapy based on postpartum thyroid function tests 7
• Return to the preconception dose that maintained euthyroidism before pregnancy 3

Critical Risks of Inadequate Treatment

Untreated or inadequately treated hypothyroidism during pregnancy carries significant maternal and fetal risks 6, 3:

• Maternal complications: Preeclampsia, placental abruption 6, 2
• Fetal complications: Low birth weight, preterm delivery 6, 3
• Neurodevelopmental effects: Impaired cognitive development in offspring, increased risk of congenital cretinism with severe iodine deficiency 6, 1
• Fetal mortality: Increased risk of fetal loss 7

Common Pitfalls to Avoid

• Waiting to adjust dose: Do not wait for TSH to become elevated before increasing levothyroxine; increase proactively at pregnancy confirmation 1, 2
• Infrequent monitoring: Checking TSH only once per trimester may miss critical periods of inadequate replacement 5
• Forgetting postpartum reduction: Continuing the pregnancy dose postpartum leads to iatrogenic hyperthyroidism with risks of atrial fibrillation and bone loss 4
• Ignoring isolated low T4: Normal TSH with low free T4 during pregnancy still requires treatment 7