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Thyroid Disorders in Pregnancy

RCOG Green-top Guideline No. 76 Management of Thyroid Disorders in Pregnancy. Last reviewed: Apr 2025.

Pre-Existing Thyroid Disorders

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.

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

Choice of anti-thyroid drugs:
  • 1st trimester: propylthiouracil
  • 2nd and 3rd trimester: carbimazole

Maintain free T4 at upper half of normal

 

ROCG recommends considering discontinuing antithyroid drugs with closing TFT monitoring if:
 

  • Euthyroid prior to conception, and
  • On low-dose antithyroid drug for ≥6 months

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 

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

  • ↑ TSH
  • ↓ Free T4

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

  • ↑ TSH (above pregnancy-specific reference range)
  • Normal free T4

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

  • Stop levothyroxine following birth
  • Check TFT 6 weeks postpartum

  • ↓ TSH
  • ↑ T4

Start antithyroid drugs:
  • 1st trimester: propylthiouracil
  • 2nd and 3rd trimester: carbimazole

Start with the lowest effective dose to maintain free T4 at upper half of normal
  • Monitor TFT every 2-4 weeks

References

Author: Adams Lau
Reviewer:
Last edited: 10/07/25