What is the recommended management for a pregnant woman with hypothyroidism (underactive thyroid gland)?

Hypothyroidism Management in Pregnancy

Immediate Levothyroxine Dose Adjustment Upon Pregnancy Confirmation

Women with pre-existing hypothyroidism must increase their levothyroxine dose by 25-50% (approximately two extra tablets per week) immediately upon pregnancy confirmation to prevent maternal hypothyroidism and protect fetal neurocognitive development. 1, 2

Critical Timing and Rationale

• Levothyroxine requirements increase as early as the fifth week of gestation, with a mean increase of 47% during the first half of pregnancy 2
• The dose increase plateaus by week 16 and remains elevated until delivery 2
• Untreated or inadequately treated maternal hypothyroidism causes spontaneous abortion, gestational hypertension, pre-eclampsia, stillbirth, premature delivery, and permanent adverse effects on fetal neurocognitive development 1
• Up to 75% of hypothyroid women require higher levothyroxine doses during pregnancy to maintain normal TSH levels 3

Specific Dosing Protocol

For Women with Pre-Existing Hypothyroidism:

• Increase pre-pregnancy levothyroxine dose by two extra tablets per week (approximately 30% increase) as soon as pregnancy is confirmed 2, 4
• This translates to increasing by 12.5-25 mcg per day 1
• For women on higher pre-pregnancy doses (≥100 mcg/day) or with TSH <1.5 mIU/L, use the lower increment (two tablets/week) to avoid TSH suppression 4

For New-Onset Hypothyroidism During Pregnancy:

• TSH ≥10 mIU/L: Start levothyroxine at 1.6 mcg/kg/day 1
• TSH <10 mIU/L: Start levothyroxine at 1.0 mcg/kg/day 1

Monitoring Schedule

• Measure TSH and free T4 as soon as pregnancy is confirmed 1
• Monitor TSH every 4 weeks until stable, then at minimum once per trimester 1, 4
• Target TSH within trimester-specific reference range, ideally <2.5 mIU/L in the first trimester 1
• Testing every 4 weeks identifies 92% of abnormal values 4

Dose Adjustment Algorithm During Pregnancy

• If TSH rises above trimester-specific range: Increase levothyroxine by 12.5-25 mcg per day 1
• Recheck TSH 4 weeks after any dose adjustment 1
• Continue adjusting until TSH normalizes within trimester-specific range 1
• The mean levothyroxine requirement during pregnancy is approximately 150 mcg/day, though individual variation is substantial 3

Postpartum Management

• Reduce levothyroxine to pre-pregnancy dose immediately after delivery 1
• Monitor TSH 4-8 weeks postpartum to confirm appropriate dosing 1
• Postpartum TSH levels typically return to preconception values 1

Critical Safety Considerations

Risks of Inadequate Treatment:

• Maternal hypothyroidism during pregnancy increases risk of preeclampsia, low birth weight, and permanent neurodevelopmental deficits in the child 5, 1
• Even subclinical hypothyroidism is associated with adverse pregnancy outcomes 5
• Fetal brain development depends critically on maternal thyroid hormone, particularly in the first and second trimesters 1

Common Pitfalls to Avoid:

• Never wait for symptoms to develop before checking TSH—fetal harm can occur before maternal symptoms appear 5
• Never discontinue levothyroxine during pregnancy, even if TSH normalizes temporarily 1
• Never delay dose increase until first prenatal visit—increase immediately upon positive pregnancy test 2, 4
• Avoid TSH targets >2.5 mIU/L in the first trimester, as even mild TSH elevation is associated with adverse outcomes 5

Special Populations

Athyreotic Patients (Post-Thyroidectomy or Radioiodine):

• These patients have higher risk of TSH suppression with standard dose increases 4
• Consider using the lower increment (two tablets/week rather than three) 4
• Monitor more frequently (every 2-4 weeks) during first trimester 4

Women Planning Pregnancy:

• Optimize thyroid function before conception with target TSH <2.5 mIU/L 5
• Counsel to increase levothyroxine dose immediately upon positive pregnancy test 2
• Ensure adequate understanding of the critical importance of early dose adjustment 1

Medication-Specific Guidance

• Levothyroxine is the only appropriate thyroid hormone replacement during pregnancy—T3 supplementation provides inadequate fetal thyroid hormone delivery 5
• Take levothyroxine on an empty stomach, 30-60 minutes before breakfast, at least 4 hours apart from iron, calcium supplements, or antacids 5
• Levothyroxine is present in breast milk but causes no adverse effects on breastfed infants 1