What are the guidelines for managing hypothyroidism (underactive thyroid) in pregnancy?

Managing Hypothyroidism in Pregnancy

For pregnant women with hypothyroidism, levothyroxine dosage should be increased by 12.5-25 mcg per day when TSH exceeds trimester-specific ranges, with monitoring every 4 weeks until TSH stabilizes within the normal trimester-specific range. 1

Initial Assessment and Monitoring

• Check TSH and free T4 as soon as pregnancy is confirmed and at minimum during each trimester 2, 1
• For women with pre-existing hypothyroidism:
  • Ideally maintain TSH below 1.2 mIU/L before conception to reduce the need for dose adjustments during pregnancy 3
  • Women with pre-conception TSH between 1.2-2.4 mIU/L have a 50% chance of requiring dose increases during pregnancy, compared to only 17.2% when pre-conception TSH is <1.2 mIU/L 3

Dosing Guidelines for Hypothyroidism in Pregnancy

Pre-existing Hypothyroidism:

• If TSH is above normal trimester-specific range:
  • Increase levothyroxine by 12.5-25 mcg per day 1
  • Monitor TSH every 4 weeks until stable dose is reached 1
  • Return to pre-pregnancy dosage immediately after delivery 1
  • Check TSH 4-8 weeks postpartum 1

Newly Diagnosed Hypothyroidism in Pregnancy:

• For TSH ≥10 IU/L: Start at 1.6 mcg/kg/day 1
• For TSH <10 IU/L: Start at 1.0 mcg/kg/day 1
• Monitor TSH every 4 weeks and adjust dosage until TSH is within trimester-specific range 1

Monitoring Schedule

• Check TSH and free T4 immediately upon pregnancy confirmation 4
• Monitor every 4 weeks throughout pregnancy 4
• More frequent monitoring (every 2-4 weeks) after dose adjustments 4
• Studies show that 43% of women have TSH values above recommended guidelines in the first trimester (>2.5 mU/L) 5

Risks of Inadequate Treatment

• Uncontrolled hypothyroidism during pregnancy is associated with:
  • Increased risk of severe preeclampsia
  • Preterm delivery
  • Low birth weight
  • Poor cognitive development in children 4

Risks of Overtreatment

• TSH <0.10 mIU/L during pregnancy increases the odds of preterm delivery (adjusted odds ratio: 2.14) 6
• Avoid overtreatment by careful monitoring and appropriate dose adjustments

Common Pitfalls and How to Avoid Them

1. Inadequate pre-conception optimization:

   • Aim for TSH <1.2 mIU/L before conception to minimize the need for dose adjustments during pregnancy 3

2. Delayed monitoring:

   • Studies show that 17.8% of women on thyroid replacement before conception do not have TSH measured during pregnancy 6
   • Ensure prompt testing as soon as pregnancy is confirmed

3. Insufficient dose adjustments:

   • Up to 75-81% of TSH values can be maintained within trimester-specific ranges with appropriate algorithms for dose adjustment 7
   • Consider either empiric dose increases (two pills/week initially) or ongoing adjustments based on current TSH levels 7

4. Failure to return to pre-pregnancy dose:

   • Reduce levothyroxine dosage to pre-pregnancy levels immediately after delivery 1
   • Monitor TSH 4-8 weeks postpartum to ensure appropriate levels 1

By following these guidelines and maintaining vigilant monitoring throughout pregnancy, the risks associated with both under-treatment and over-treatment of hypothyroidism can be minimized, leading to better maternal and fetal outcomes.