At 33 weeks gestation with a thyroid‑stimulating hormone (TSH) level of 4 mIU/L while taking levothyroxine, should the dose be increased?

Levothyroxine Dose Adjustment at 33 Weeks Gestation with TSH 4 mIU/L

Yes, increase the levothyroxine dose immediately—pregnancy increases thyroid hormone requirements by 25-50%, and a TSH of 4 mIU/L at 33 weeks indicates inadequate replacement that requires prompt correction to prevent adverse maternal and fetal outcomes. 1, 2

Rationale for Immediate Dose Increase

• Levothyroxine requirements increase as early as the fifth week of gestation, with a mean increase of 47% during the first half of pregnancy, plateauing by week 16 and remaining elevated until delivery 2
• At 33 weeks gestation, a TSH of 4 mIU/L is above the pregnancy-specific target and indicates insufficient thyroid hormone replacement 1, 2
• Untreated or inadequately treated maternal hypothyroidism increases risk of preeclampsia, gestational hypertension, stillbirth, premature delivery, and adverse effects on fetal neurocognitive development 1, 3

Specific Dose Adjustment Protocol

• Increase the current levothyroxine dose by 25-50 mcg immediately (typically a 25-30% increase from the pre-pregnancy dose) 1, 2
• If the pre-pregnancy dose is known, increase by approximately 30% as this represents the typical requirement increase during pregnancy 2
• Recheck TSH and free T4 in 4 weeks after dose adjustment to ensure adequate response, then continue monitoring monthly throughout the remainder of pregnancy 1, 2

Target TSH During Pregnancy

• Maintain TSH within trimester-specific reference ranges, ideally <2.5 mIU/L in the first trimester and within the lower half of the normal range throughout pregnancy 1, 4
• Women with preconception TSH >1.2 mIU/L have a 50% likelihood of requiring dose increases during pregnancy, compared to only 17% when preconception TSH is <1.2 mIU/L 4
• The goal is to maintain maternal euthyroidism throughout pregnancy to ensure normal fetal cognitive development 2

Critical Safety Considerations

• Levothyroxine should never be discontinued during pregnancy—hypothyroidism diagnosed or inadequately treated during pregnancy must be promptly corrected 3
• Serum TSH levels should be monitored and levothyroxine dosage adjusted throughout pregnancy, as requirements continue to increase 3
• After delivery, return to the pre-pregnancy levothyroxine dose immediately, as postpartum TSH levels revert to preconception values 3

Common Pitfalls to Avoid

• Do not wait for symptoms to develop before adjusting the dose—fetal harm can occur before maternal symptoms appear, and maintaining biochemical euthyroidism is essential 1
• Avoid undertitrating the dose increase—a 12.5 mcg increment is typically insufficient during pregnancy when requirements increase by 25-50% 1, 2
• Never delay dose adjustment in the third trimester—adequate thyroid hormone is critical throughout all stages of pregnancy for fetal brain development 1, 2