What is the recommended management for hypothyroidism (underactive thyroid) in pregnant women?

Management of Hypothyroidism During Pregnancy

Pregnant women with hypothyroidism should be treated with levothyroxine to maintain TSH within trimester-specific reference ranges and Free T4 in the upper normal range to reduce risks of maternal and fetal complications. 1, 2

Diagnosis and Assessment

• Initial evaluation should include TSH and Free T4 (or Free Thyroxine Index) testing for pregnant women with suspected hypothyroidism 1
• Normal TSH reference ranges during pregnancy are:
  • First trimester: 0.1-2.5 mIU/L 3
  • Second trimester: 0.2-3.0 mIU/L 3
  • Third trimester: 0.3-3.0 mIU/L 3
• Even isolated maternal hypothyroxinemia (low T4 with normal TSH) during pregnancy requires treatment due to potential risks to fetal neuropsychological development 2

Treatment Approach

For Women Already on Levothyroxine Before Pregnancy:

• Increase levothyroxine dose by 30-50% as soon as pregnancy is confirmed 4, 5
• Women with pre-existing hypothyroidism often require dosage increases during the first trimester 6, 7
• Target TSH value of 1 mIU/L before conception is recommended 6

For Newly Diagnosed Hypothyroidism During Pregnancy:

• Start levothyroxine immediately at 1.6 mcg/kg/day for TSH ≥10 IU/L 4
• Start levothyroxine at 1.0 mcg/kg/day for TSH <10 IU/L 4
• For overt hypothyroidism diagnosed during pregnancy, starting doses of 100-150 mcg daily can be considered safe to rapidly achieve euthyroidism 5

Monitoring and Dose Adjustment

• Monitor thyroid function every 4 weeks until stable, then every trimester 1, 2, 4
• Adjust levothyroxine dose to maintain:
  • TSH within trimester-specific reference ranges 4, 3
  • Free T4 in the upper half of the normal range 2, 4
• Up to 75% of women taking levothyroxine require higher doses during pregnancy to maintain normal TSH levels 7

Clinical Considerations and Risks

• Untreated maternal hypothyroidism is associated with:
  • Increased risk of preeclampsia 1
  • Low birth weight in neonates 1, 3
  • Fetal distress 3
  • Impaired neuropsychological development in the child 2, 6, 3
  • Increased risk of spontaneous abortion, gestational hypertension, stillbirth, and premature delivery 4

Postpartum Management

• Reduce levothyroxine dose to pre-pregnancy levels immediately after delivery 4, 7
• Monitor serum TSH 4-8 weeks postpartum 4
• Reassess thyroid function at 6-12 weeks postpartum 7

Special Considerations

• Adequate levothyroxine treatment during lactation may normalize milk production in hypothyroid mothers with low milk supply 4
• Levothyroxine is present in human milk but no adverse effects on breastfed infants have been reported 4
• The developmental benefits of breastfeeding outweigh potential risks for mothers requiring levothyroxine 4

Common Pitfalls to Avoid

• Failing to increase levothyroxine dose early in pregnancy, which can lead to maternal hypothyroidism and adverse fetal outcomes 5
• Not monitoring thyroid function frequently enough during pregnancy (every 4 weeks until stable) 2, 4
• Overlooking isolated hypothyroxinemia (low T4 with normal TSH), which still requires treatment during pregnancy 2
• Not reducing levothyroxine dose back to pre-pregnancy levels after delivery 4, 7