Thyroid hormone is essential for fetal brain development — which is why thyroid health before and during pregnancy matters more than most people realise.
Before pregnancy: the preconception check
If you have known thyroid disease or risk factors, preconception testing is recommended:
- Known hypothyroidism — dose requirements increase ~30% in pregnancy; optimise before conceiving
- Positive anti-TPO antibodies — even with normal function, they increase miscarriage and preterm risk
- Previous postpartum thyroiditis — recurrence risk in future pregnancies is significant
- Family history — consider baseline screening
A TSH in the normal range before pregnancy gives the best starting point.
During pregnancy: tighter targets
Pregnancy changes thyroid requirements dramatically:
- First trimester — the fetus depends entirely on maternal thyroid hormone until ~12 weeks
- TSH targets are stricter — trimester-specific ranges apply (typically <2.5 in first trimester)
- Dose increases — levothyroxine requirements rise; most hypothyroid women need dose adjustment
- Monitoring — TSH checked every 4–6 weeks during pregnancy
Untreated or under-treated hypothyroidism in pregnancy increases risks of miscarriage, preterm birth, low birth weight, and impaired fetal development.
After delivery: postpartum thyroiditis
Up to 10% of women develop postpartum thyroiditis within the first year — inflammation causing a hyperthyroid phase followed by hypothyroidism. It is more common in women with anti-TPO antibodies or other autoimmune conditions.
Symptoms can be mistaken for normal postpartum fatigue — but persistent or unusual symptoms deserve testing.
The bottom line
Thyroid health in pregnancy is not optional — it directly affects fetal development. If you are planning pregnancy, currently pregnant, or recently delivered, thyroid function should be checked and managed appropriately. Early detection and treatment protect both mother and baby.