What is the appropriate starting dose of levothyroxine for a 27‑week pregnant woman with overt hypothyroidism?

Starting Dose of Levothyroxine in Overt Hypothyroidism at 27 Weeks Pregnancy

For a 27-week pregnant woman with newly diagnosed overt hypothyroidism, start levothyroxine at 1.6 mcg/kg/day based on pre-pregnancy or current body weight immediately, and recheck TSH and free T4 in 4 weeks. 1

Critical Pre-Treatment Safety Assessment

Before initiating levothyroxine, you must rule out concurrent adrenal insufficiency by measuring morning cortisol and ACTH, as starting thyroid hormone before adequate corticosteroid coverage can precipitate life-threatening adrenal crisis 2. If adrenal insufficiency is present or suspected, initiate hydrocortisone for at least one week before starting levothyroxine 2.

Initial Dosing Strategy

The FDA-approved starting dose for new-onset hypothyroidism in pregnancy is 1.6 mcg/kg/day 1. This full replacement dose is appropriate because:

• Untreated maternal hypothyroidism at 27 weeks carries unacceptable risks of preeclampsia, low birth weight, and permanent neurodevelopmental deficits in the child 2
• Fetal brain development depends critically on maternal thyroid hormone, particularly in the second and third trimesters 2
• Delayed treatment or underdosing increases risk of adverse pregnancy outcomes including gestational hypertension, stillbirth, and premature delivery 2

For a typical pregnant woman weighing 70 kg, this translates to approximately 112 mcg daily (round to 100-125 mcg) 1, 3. Use actual body weight for dose calculation, not ideal body weight 3, 4.

Monitoring Protocol During Pregnancy

Recheck TSH and free T4 every 4 weeks until stable, then at minimum once per trimester 1. The target is:

• TSH within trimester-specific reference range, ideally <2.5 mIU/L 2, 1
• Free T4 in the upper half of the normal range 2

Adjust levothyroxine by 12.5-25 mcg increments based on TSH results 2, 1. Approximately 25-50% dose increases are typically needed during pregnancy in women with pre-existing hypothyroidism, though this patient has new-onset disease 2, 1.

Special Considerations at 27 Weeks Gestation

At 27 weeks, you are in the third trimester where:

• Levothyroxine requirements are typically at their peak during pregnancy 2
• The fetus still depends substantially on maternal thyroid hormone for neurodevelopment 2
• Rapid normalization of maternal thyroid function is essential to prevent ongoing fetal harm 2

Do not use a conservative "start low, go slow" approach in this clinical scenario—the standard full replacement dose of 1.6 mcg/kg/day is appropriate and necessary 1, 5.

Critical Pitfalls to Avoid

• Never delay treatment to "confirm" the diagnosis with repeat testing—overt hypothyroidism in pregnancy requires immediate treatment 2
• Do not start with 25-50 mcg/day as you would in elderly patients with cardiac disease; pregnancy is a unique situation requiring full replacement immediately 2, 5
• Avoid waiting for symptoms to develop before checking TSH, as fetal harm can occur before maternal symptoms appear 2
• Never use combination T4/T3 therapy during pregnancy—levothyroxine monotherapy is the only appropriate treatment as T3 does not adequately cross the placenta 2

Patient Education Points

Instruct the patient to:

• Take levothyroxine on an empty stomach, 30-60 minutes before breakfast, for optimal absorption 2, 3
• Maintain at least 4 hours separation from iron, calcium supplements, or antacids 2, 3
• Understand that thyroid hormone is critical for fetal brain development and treatment cannot be delayed 2
• Report any symptoms of overtreatment (palpitations, tremor, heat intolerance) or undertreatment (persistent fatigue, cold intolerance) 2

Post-Delivery Management

Reduce levothyroxine dose to pre-pregnancy levels immediately after delivery and monitor serum TSH 4-8 weeks postpartum 1. Many women with pregnancy-induced hypothyroidism may not require lifelong treatment, so reassessment postpartum is essential 2.