What is the recommended management of hypothyroidism (underactive thyroid) in pregnancy?

Management of Hypothyroidism in Pregnancy

Hypothyroidism in pregnant women should be treated with levothyroxine in a sufficient dosage to return the TSH level to normal, with dose adjustments every four weeks until the TSH level is stable and monitoring every trimester. 1

Diagnosis and Initial Assessment

• Check thyroid-stimulating hormone (TSH) and free T4 levels as soon as pregnancy is confirmed
• For pregnant women with pre-existing hypothyroidism, maintain serum TSH in the trimester-specific reference range 2
• Untreated maternal hypothyroidism is associated with:
  • Increased risk of spontaneous abortion, gestational hypertension, pre-eclampsia, stillbirth, and premature delivery 2
  • Potential adverse effects on fetal neurocognitive development 2, 3

Levothyroxine Dosing Algorithm

Pre-existing Hypothyroidism

1. Immediate dose increase: Increase levothyroxine dose by approximately 30% as soon as pregnancy is confirmed 3
2. Monitoring: Check TSH every 4 weeks until stable, then each trimester 1, 2
3. Dose adjustments: Adjust dose by 12.5-25 mcg increments to maintain TSH within trimester-specific range 2
4. Post-delivery: Return to pre-pregnancy dose immediately after delivery 2

Newly Diagnosed Hypothyroidism During Pregnancy

• For TSH ≥10 IU/L (overt hypothyroidism): Start at 1.6 mcg/kg/day 2
  • Alternatively, 2.33 mcg/kg/day has been shown effective 4
• For TSH <10 IU/L (subclinical hypothyroidism): Start at 1.0 mcg/kg/day 2
  • For TSH >4.2-10 mIU/L: Consider 1.42 mcg/kg/day 4
  • For TSH ≤4.2 mIU/L: Consider 1.20 mcg/kg/day 4

Timing of Levothyroxine Requirement Changes

• Levothyroxine requirements increase as early as the fifth week of gestation 3
• The mean levothyroxine requirement increases by 47% during the first half of pregnancy 3
• Requirements typically plateau by week 16 of gestation 3
• Up to 75% of women with hypothyroidism require higher doses during pregnancy 5

Monitoring Protocol

• First trimester: Check TSH approximately every 4 weeks 2, 5
• Second and third trimesters: Check TSH monthly or at least once per trimester 1, 5
• Target TSH levels:
  • First trimester: ≤2.5 mIU/L 4
  • Second and third trimesters: ≤3.0 mIU/L 4

Special Considerations

• Fixed dosing approaches (e.g., 50 mcg/day) are inadequate for many pregnant women with hypothyroidism 6
• Weight-based dosing is more effective at achieving target TSH levels 4
• The mean levothyroxine dose needed during pregnancy is approximately 150 mcg/day, but individual requirements vary widely 5

Postpartum Management

• Reduce levothyroxine dose to pre-pregnancy levels immediately after delivery 2, 3
• Reassess thyroid function 6-12 weeks postpartum 5
• Monitor for postpartum thyroiditis, especially in women with positive thyroid antibodies 1

Common Pitfalls to Avoid

1. Delayed dose adjustment: Failure to increase levothyroxine dose early in pregnancy can lead to maternal and fetal complications
2. Inadequate monitoring: Not checking TSH frequently enough during pregnancy
3. Insufficient initial dosing: Starting with too low a dose when hypothyroidism is newly diagnosed in pregnancy
4. Failure to reduce dose postpartum: Not returning to pre-pregnancy dose after delivery
5. Using fixed doses: Not individualizing therapy based on TSH levels and body weight

By following this algorithm, clinicians can effectively manage hypothyroidism during pregnancy, minimizing risks to both mother and fetus.