How does pregnancy affect thyroid‑stimulating hormone levels in a woman with primary hypothyroidism who is stable on levothyroxine?

TSH Rises During Pregnancy in Women with Hypothyroidism on Levothyroxine

Women with hypothyroidism who are stable on levothyroxine will experience a significant increase in TSH during early pregnancy—often as early as 5 weeks gestation—requiring immediate dose adjustment to prevent maternal and fetal complications. 1

Why TSH Increases During Pregnancy

• Thyroid hormone requirements increase by 25-50% during pregnancy due to increased thyroid-binding globulin production, increased maternal blood volume, and transfer of thyroid hormone to the fetus 1, 2

• The rise in TSH begins as early as the 5th week of gestation, with the median onset at 8 weeks, and plateaus by week 16 1

• Without dose adjustment, 17 of 20 pregnancies (85%) in one prospective study required levothyroxine increases to maintain euthyroidism 1

Immediate Management Upon Pregnancy Confirmation

Increase levothyroxine by 25-30% (approximately two extra tablets per week) as soon as pregnancy is confirmed, before waiting for TSH results 1, 3, 2

• This proactive approach prevents TSH elevation above 5.0 mIU/L throughout the first trimester and replicates normal physiological changes 3

• Delaying dose adjustment until TSH rises puts the fetus at risk for impaired neurocognitive development, as maternal thyroid hormone is critical for fetal brain development in the first trimester 1, 2

Monitoring Protocol During Pregnancy

• Check TSH and free T4 every 4 weeks through midgestation, then at 30 weeks gestation 3, 2

• Target TSH <2.5 mIU/L in the first trimester and <3.0 mIU/L in the second and third trimesters 2, 4

• Monitoring every 4 weeks identifies 92% of abnormal values and allows timely dose adjustments 3

Risk Factors for Excessive TSH Suppression

Certain patients are at higher risk of over-suppression when increasing levothyroxine by 30%: 3

• Athyreotic patients (post-thyroidectomy or radioiodine ablation) have 3.3-fold increased risk of TSH suppression below 0.5 mIU/L 3

• Pre-pregnancy TSH <1.5 mIU/L increases risk 4.6-fold 3

• Pre-pregnancy levothyroxine dose ≥100 mcg/day increases risk 7.2-fold 3

For these high-risk patients, consider a more conservative increase of two tablets per week (approximately 20-25% increase) rather than 30% 3

Consequences of Inadequate Treatment

• Untreated or inadequately treated maternal hypothyroidism increases risk of: 1, 2

  • Preeclampsia 5
  • Low birth weight 5
  • Increased fetal mortality 1
  • Permanent neurodevelopmental deficits in the child, particularly affecting cognitive development 1, 2

• The critical window for fetal brain development is the first trimester, when the fetus is entirely dependent on maternal thyroid hormone 1, 2

Postpartum Management

• Levothyroxine requirements return to pre-pregnancy levels after delivery 1

• Return to the pre-pregnancy dose immediately postpartum and recheck TSH in 6-8 weeks 5

Common Pitfalls to Avoid

• Never wait for TSH results before increasing levothyroxine in a woman with known hypothyroidism who confirms pregnancy—the dose increase should be immediate 1, 3

• Do not use the standard non-pregnant TSH reference range (0.45-4.5 mIU/L) during pregnancy; pregnancy-specific targets are lower 2, 4

• Avoid under-dosing in the first trimester, as this is when fetal neurodevelopment is most vulnerable to maternal hypothyroidism 1, 2

• Do not assume a single dose adjustment will suffice—more than 50% of women require additional adjustments during pregnancy, necessitating regular monitoring 2