Implications / Complications
| Scenario | Maternal risk | Fetal / newborn risk |
|---|---|---|
| Untreated / undertreated hypothyroidism | Increased risk of:
|
Increased risk of:
Major implication: impaired fetal neurodevelopment and lower IQ |
| Untreated / poorly controlled hyperthyroidism | Increased risk of:
|
Increased risk of:
In contrast, treating maternal hyperthyroidism can risk neonatal hypothyroidism (as the drug can cross the placenta) |
Pre-Existing Thyroid Disorders
Graves’ Hyperthyroidism
Pre-Conception
Discuss the option of definitive treatment with radioactive iodine or thyroidectomy prior to 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.
Early Pregnancy Management
| Management aspect | Key principles |
|---|---|
| Anti-thyroid drugs | In 1st trimester: propylthiouracil is preferred
In 2nd and 3rd trimester: switch back to carbimazole
|
| Tests and monitoring | Monitor:
Treatment aim: maintain free T4 at the upper half of normal |
| Fetal monitoring | Serial ultrasound with umbilical artery Doppler monthly at 26-28 weeks is recommended if:
|
ROCG recommends considering discontinuing antithyroid drugs with close TFT monitoring if:
- Euthyroid prior to conception, and
- On low-dose antithyroid drug for ≥6 months
Hypothyroidism
Pre-Conception
Aim for TSH ≤2.5 mU/L before conception.
Management During Pregnancy
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)
Postpartum Management
Following birth: revert to pre-conception dose of levothyroxine at 2 weeks postpartum
Newly Diagnosed Thyroid Disorders During Pregnancy
Overt Hypothyroidism
Definition
- ↑ TSH
- ↓ Free T4
Management During Pregnancy
Start levothyroxine immediately and continue throughout pregnancy
- 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
Postpartum Management
- Stop levothyroxine following birth
- Check TFT 6 weeks postpartum to determine if long-term treatment is necessary
Subclinical Hypothyroidism
Definition
- ↑ TSH (above pregnancy-specific reference range)
- Normal free T4
Management During Pregnancy
Management depends on TSH level:
| TSH level (mU/L) | Management |
|---|---|
| >10 | Start levothyroxine immediately (treat as overt hypothyroidism) |
| <10 | 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 |
Postpartum Management
- Stop levothyroxine following birth
- Check TFT 6 weeks postpartum to determine if long-term treatment is necessary
Hyperthyroidism
Definition
- ↓ TSH
- ↑ T4
Management During Pregnancy
Initiate antithyroid drugs, choice of drug depends on when the diagnosis is made:
- 1st trimester: propylthiouracil (if still necessary beyond 20 weeks → switch to carbimazole)
- 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 during the first half of the pregnancy
Because maternal thyroid autoimmunity naturally subsides as the pregnancy progresses, many women will be able to gradually reduce their dose.
Most women can safely discontinue treatment entirely in the late second or early third trimester.
Radioactive iodine should be avoided during pregnancy.
Surgery is rarely performed during pregnancy; it is reserved for severe cases, such as when a woman has severe adverse reactions to antithyroid drugs or a large goitre that could compromise the airway.
Postpartum Management
Maternal management:
- Check TFT 6-8 weeks after birth
- If treatment is still required, both carbimazole and propylthiouracil are safe for breastfeeding (use the lowest effective dose during lactation)
The newborn must have their TFTs monitored soon after birth and again at 1–2 weeks post-birth
- This is critical to check for neonatal hyperthyroidism (caused by the mother’s stimulating antibodies crossing the placenta) or neonatal hypothyroidism (caused by maternal antithyroid drug treatment)