What are the goals for managing thyroid levels in pregnancy?

Managing Thyroid Levels in Pregnancy

The primary goal for managing thyroid levels in pregnancy is to maintain TSH within trimester-specific reference ranges (0.1-2.5 mIU/L in first trimester, 0.2-3.0 mIU/L in second trimester, and 0.3-3.0 mIU/L in third trimester) to prevent adverse maternal and fetal outcomes. 1, 2

Screening and Monitoring

• Target population for screening: Women with:

  • Family or personal history of thyroid disease
  • Symptoms of goiter or hypothyroidism
  • Type 1 diabetes mellitus
  • Personal history of autoimmune disorders 1

• Monitoring protocol:

  • Initial test: TSH (with Free T4 or Free T4 Index for suspected thyroid dysfunction)
  • Frequency: As soon as pregnancy is confirmed, then at minimum once each trimester
  • For women on treatment: Every 4-6 weeks until TSH levels stabilize 1, 3

Management of Hypothyroidism

Treatment Goals

• For pre-existing hypothyroidism: Maintain TSH in trimester-specific reference range
• For new-onset hypothyroidism: Initiate treatment promptly to normalize thyroid function 1, 3

Treatment Protocol

1. Medication: Levothyroxine is the treatment of choice
2. Dosing adjustments:
   • Pre-existing hypothyroidism: Typically need 30% or more dose increase by 4-6 weeks gestation
   • For TSH above normal trimester-specific range: Increase by 12.5-25 mcg/day
   • New-onset hypothyroidism (TSH ≥10 IU/L): 1.6 mcg/kg/day
   • New-onset hypothyroidism (TSH <10 IU/L): 1.0 mcg/kg/day 1, 3
3. Post-delivery: Reduce levothyroxine dosage to pre-pregnancy levels immediately after delivery
4. Post-partum monitoring: Check TSH 4-8 weeks postpartum 3

Management of Hyperthyroidism

Treatment Goals

• Maintain Free T4 in high-normal range using lowest possible medication dose 1

Treatment Protocol

1. First trimester: Propylthiouracil preferred (lower risk of teratogenicity)
2. Second and third trimesters: Methimazole preferred (lower risk of hepatotoxicity)
3. Monitoring: Free T4 every 2-4 weeks
4. Breastfeeding: Both propylthiouracil and methimazole considered safe 1

Risks of Untreated Thyroid Disease

Maternal Risks

• Heart failure
• Spontaneous abortion
• Preterm birth
• Stillbirth
• Impaired cardiac and metabolic function 1

Fetal/Neonatal Risks

• Low birth weight
• Fetal distress
• Cognitive impairment
• Placental abruption 1, 2

Special Considerations

Postpartum Thyroiditis (PPT)

• Affects 5-10% of women within first year after delivery
• Treatment:
  • Hyperthyroid phase: Beta-blockers for symptomatic relief
  • Hypothyroid phase: Levothyroxine for symptomatic patients or TSH >10 mIU/L
• Long-term follow-up needed as 20-40% develop permanent hypothyroidism 1

Common Pitfalls to Avoid

1. Using non-pregnancy reference ranges: Pregnancy-specific TSH reference ranges are essential for proper diagnosis and management 2, 4

2. Delayed treatment: Treatment of overt hypothyroidism should begin immediately, as fetal brain development may be irreversibly affected after 14 weeks gestation 5

3. Inadequate monitoring: Regular monitoring is crucial as thyroid requirements change throughout pregnancy 1, 3

4. Radioactive iodine use: Absolutely contraindicated during pregnancy and breastfeeding 1

5. Overlooking subclinical hypothyroidism: Even subclinical hypothyroidism should be treated during pregnancy to prevent adverse outcomes 1