Thyroid Disorders in Pregnancy
RCOG Green-top Guideline No. 76 Management of Thyroid Disorders in Pregnancy. Last reviewed: Apr 2025.
Pre-Existing Thyroid Disorders
Graves’ Hyperthyroidism
Pre-Conception Counselling
Discuss the option of definitive treatment with radioactive iodine or thyroidectomy PRIOR conception, especially in those with severe disease.
Following definitive treatment, wait at least 6 months before attempting to conceive and only when 2 measurements (3 months apart) are within the reference range.
Tests
Monitor TFT every 2-4 weeks (4-8 weekly testing after 20 weeks)
TSH receptor antibody level should be measured in 1st trimester (ALL women with history of Graves’, even after definitive management)
- If elevated / woman is taking antithyroid drugs → further measurement at 20 and 28 weeks
Anti-Thyroid Drugs
Choice of anti-thyroid drugs:
- 1st trimester: propylthiouracil
- 2nd and 3rd trimester: carbimazole
Maintain free T4 at the upper half of normal
ROCG recommends considering discontinuing antithyroid drugs with close TFT monitoring if:
- Euthyroid prior to conception, and
- On low-dose antithyroid drug for ≥6 months
Fetal Monitoring
Serial ultrasound with umbilical artery Doppler monthly at 26-28 weeks is recommended if:
- Required antithyroid drug treatment
- Uncontrolled Graves’ disease at any time during pregnancy
- TSH receptor antibody level 3x above threshold
Hypothyroidism
Aim TSH ≤2.5 mU/L before conception.
Upon pregnancy confirmation → increase the existing levothyroxine dose by 25-30%
- Double the dose on 2 days of each week, or
- Increase 25 mcg per day (if taking <100 mcg) or 50 mcg per day (if taking >100 mcg)
Following birth: revert to pre-conception dose of levothyroxine 2 weeks postpartum
Newly Diagnosed Thyroid Disorders
Overt Hypothyroidism
Definition
- ↑ TSH
- ↓ Free T4
Management
Start levothyroxine immediately and continue throughout pregnancy and postpartum:
- Initial dose: 1.6 mcg/kg/day
- Aim: TSH ≤2.5 mU/L
- TFT every 4-6 weeks until 20 weeks, then repeat once at 28 weeks
Subclinical Hypothyroidism
Definition
- ↑ TSH (above pregnancy-specific reference range)
- Normal free T4
Antenatal Management
Management depends on TSH level:
- TSH >10 mU/L → start levothyroxine immediately (treat as overt hypothyroidism)
- TSH <10 mU/L →
- Consider levothyroxine treatment (especially if +ve anti-TPO antibodies or IVF pregnancy), or
- No treatment + TFT every 4-6 weeks until 20 weeks, then again at 28 weeks
Postnatal Management
- Stop levothyroxine following birth
- Check TFT 6 weeks postpartum
Hyperthyroidism
Definition
- ↓ TSH
- ↑ T4
Management
Start antithyroid drugs:
- 1st trimester: propylthiouracil
- 2nd and 3rd trimester: carbimazole
Start with the lowest effective dose to maintain free T4 at the upper half of normal
- Monitor TFT every 2-4 weeks