PTU Dosage for Pregnant Patients
Propylthiouracil (PTU) should be used at the lowest effective dose to maintain maternal free T4 in the upper normal to slightly elevated range during pregnancy, typically requiring 50-300 mg daily in divided doses, with most patients controlled on 100-200 mg daily.
Dosing Strategy
The goal is to use the minimum PTU dose necessary to control maternal hyperthyroidism while minimizing fetal thyroid suppression. There is no fixed "standard" dose—dosing must be titrated based on maternal thyroid function tests 1.
Practical Dosing Approach
- Starting doses typically range from 50-150 mg twice or three times daily, depending on severity of hyperthyroidism
- Maintenance doses commonly fall between 100-200 mg daily in divided doses
- Maximum doses up to 300-750 mg daily have been studied and used when necessary to control severe disease 2
- Doses should be adjusted to maintain maternal free T4 in the upper normal range, not to normalize TSH (which remains suppressed in pregnancy) 1
Trimester-Specific Considerations
PTU is the preferred antithyroid drug during the first trimester due to lower teratogenic risk compared to methimazole 1, 3. A 2023 meta-analysis confirmed that pregnant women treated with methimazole had significantly higher risk of congenital anomalies (OR 0.80,95% CI 0.69-0.92, P=0.002) compared to PTU 3.
Switching to methimazole in the second and third trimesters is recommended to reduce the risk of PTU-associated hepatotoxicity with prolonged use 1. The conversion ratio is approximately 1:20 (PTU:methimazole), meaning 100 mg PTU ≈ 5 mg methimazole 1.
Monitoring and Dose Adjustment
- Maternal thyroid function should be checked every 2-4 weeks and doses adjusted accordingly
- Fetal thyroid monitoring via ultrasound assessment for goiter should be performed, as fetal hypothyroidism can occur even with low maternal doses 4
- Studies show that even low doses (≤100 mg PTU daily) were associated with elevated fetal TSH in 21% of cases, while individualized higher doses sometimes resulted in normal fetal thyroid function 5
Important Caveats
The relationship between maternal PTU dose and fetal thyroid suppression is not linear or predictable 5. A 1997 study found little correlation between maternal doses and fetal thyroid status, with fetal hypothyroidism occurring even at low doses (≤100 mg daily) in some cases 5.
Fetal/neonatal hypothyroidism with or without goiter occurs in approximately 9.5% of PTU-exposed pregnancies, though most cases resolve spontaneously within the first month of life 4. Goiters detected prenatally can be successfully treated in utero by maternal dose adjustment 4.
Avoid uniformly low dosing strategies—the goal is individualized dosing that adequately controls maternal disease while monitoring for fetal effects, not arbitrarily restricting doses 5.
Lactation Dosing
PTU doses up to 750 mg daily are compatible with breastfeeding without adverse effects on infant thyroid function 2. The milk/serum concentration ratio for PTU is low (0.1), making it preferable to methimazole during lactation 6. Doses up to 150 mg daily have been specifically studied with serial infant thyroid monitoring showing normal TSH and T4 levels 6.