What are the management targets for hypothyroidism (underactive thyroid) in pregnancy?

Management Targets for Hypothyroidism in Pregnancy

For pregnant women with hypothyroidism, TSH should be maintained within trimester-specific reference ranges, with TSH ideally below 1.2 mIU/L before conception and dose adjustments of 12.5-25 mcg/day when TSH exceeds these ranges. 1, 2

TSH Targets and Monitoring

• Pre-conception: TSH should ideally be maintained below 1.2 mIU/L 1
• During pregnancy: Maintain TSH within trimester-specific reference ranges 2
• Monitoring frequency:
  • Check TSH and free T4 immediately upon pregnancy confirmation 1
  • Monitor every 4 weeks throughout pregnancy 1
  • More frequent monitoring (every 2-4 weeks) after dose adjustments 1

Levothyroxine Dosing Considerations

Initial Dosing

• Women with pre-existing hypothyroidism typically require an increase in their levothyroxine dose during pregnancy 3, 4
• The FDA label indicates that for new onset hypothyroidism with TSH ≥10 IU/L, start at 1.6 mcg/kg/day 2
• For new onset hypothyroidism with TSH <10 IU/L, start at 1.0 mcg/kg/day 2

Dose Adjustments

• Increase levothyroxine dosage by 12.5-25 mcg/day when TSH exceeds trimester-specific ranges 1, 2
• Research shows that levothyroxine requirements increase as early as the fifth week of gestation 4
• The mean increase in levothyroxine requirement is approximately 47% during the first half of pregnancy, with most increases occurring by week 8 and plateauing by week 16 4
• Some experts recommend increasing the levothyroxine dose by approximately 30% as soon as pregnancy is confirmed 4

Clinical Implications of Inadequate Treatment

Untreated or inadequately treated maternal hypothyroidism is associated with:

• Increased risk of preeclampsia 3
• Preterm delivery 1
• Low birth weight in neonates 3, 1
• Poor cognitive development in children 1
• Congenital cretinism (with iodine deficiency) 3

Post-Delivery Management

• Reduce levothyroxine dosage to pre-pregnancy levels immediately after delivery 2
• Monitor serum TSH 4-8 weeks postpartum 2

Important Considerations and Pitfalls

Factors Affecting Levothyroxine Requirements

• Even women with well-controlled hypothyroidism (pre-conception TSH <2.5 mIU/L) may require dose adjustments during pregnancy 5
• Women with pre-conception TSH between 1.2-2.4 mIU/L have a 50% chance of requiring increased levothyroxine during pregnancy, compared to only 17.2% of those with pre-conception TSH <1.2 mIU/L 5

Common Pitfalls

• Delayed recognition: Failure to check thyroid function immediately upon pregnancy confirmation
• Inadequate monitoring: Not checking TSH frequently enough during pregnancy
• Insufficient dose adjustment: Not increasing levothyroxine dose early enough or by an adequate amount
• Medication interactions: Many medications can affect levothyroxine absorption or metabolism, including calcium supplements, iron supplements, and antacids 1
• Inconsistent administration: Taking levothyroxine with food can lead to variable absorption 1

By maintaining strict control of maternal thyroid function throughout pregnancy, the risks of adverse maternal and fetal outcomes can be significantly reduced.