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

Treatment of Hypothyroidism in Pregnancy

Pregnant women with hypothyroidism require levothyroxine treatment with increased dosing requirements and frequent monitoring throughout pregnancy to maintain TSH within trimester-specific reference ranges. 1

Diagnosis and Initial Assessment

• Confirm hypothyroidism with TSH and free T4 measurement before initiating or adjusting therapy
• Normal TSH reference ranges differ during pregnancy:
  • First trimester: 0.1-2.5 mIU/L
  • Second trimester: 0.2-3.0 mIU/L
  • Third trimester: 0.3-3.0 mIU/L

Treatment Recommendations

For Women Already on Levothyroxine Before Pregnancy:

• Increase levothyroxine dose by approximately 30% as soon as pregnancy is confirmed 1, 2
• Most women (84%) will require dose adjustments during pregnancy 3
• The mean levothyroxine requirement increases by:
  • 47-50% during first trimester (median onset at 8 weeks) 3, 2
  • 55% in second trimester 3
  • 62% in third trimester 3

For Newly Diagnosed Hypothyroidism During Pregnancy:

• Start levothyroxine immediately at appropriate dosing
• Initial dose: 0.5-1.5 μg/kg/day 1
• Take as a single daily dose on an empty stomach, 30-60 minutes before breakfast 1

Monitoring Protocol

• Check TSH and free T4 every 4-6 weeks during the first half of pregnancy 1, 2
• After stable levels are achieved, monitor once each trimester 1, 4
• Adjust levothyroxine dose in 12.5-25 mcg increments to maintain TSH within trimester-specific ranges 1

Postpartum Management

• Return to pre-pregnancy levothyroxine dose immediately after delivery 5
• Reassess thyroid function 6-12 weeks postpartum 4

Important Considerations and Potential Pitfalls

• Untreated maternal hypothyroidism is associated with serious complications:

  • Increased risk of spontaneous abortion
  • Gestational hypertension
  • Pre-eclampsia
  • Stillbirth
  • Premature delivery
  • Adverse effects on fetal neurocognitive development 5, 3

• Drug Interactions:

  • Avoid taking levothyroxine with:
    • Iron supplements
    • Calcium supplements
    • Prenatal vitamins (separate by at least 4 hours)
    • Soybean products, walnuts, dietary fiber (may decrease absorption) 5

• Special Situations:

  • Monitor diabetic patients closely as levothyroxine may worsen glycemic control 5
  • Be aware that rarely, Hashimoto's hypothyroidism can convert to Graves' disease during pregnancy 6

Clinical Pearls

• The increased levothyroxine requirement begins as early as the 5th week of gestation 2
• Requirements plateau around week 16 of pregnancy 2
• Individual dose requirements vary widely - some women may need no change or even a decrease in dosage 3, 7
• The mean levothyroxine dose needed during pregnancy is approximately 150 μg/day, but this varies significantly between patients 4

By following these guidelines, clinicians can optimize thyroid hormone replacement during pregnancy, minimizing the risks associated with maternal hypothyroidism and ensuring the best possible outcomes for both mother and baby.