Levothyroxine

• Pre-existing hypothyroidism
• Subclinical hypothyroidism with TPO antibodies

Replaces endogenous T4 → converted to active T3 → nuclear receptor signalling → metabolic regulation.

• Increase dose ~25–30% as soon as pregnancy confirmed
• Recheck TSH at 6 weeks (and q4–6 weekly)
• Target TSH <2.5 mIU/L T1; <3 mIU/L T2/T3
• Return to pre-pregnancy dose immediately postpartum

Pre-existing

↑ existing dose by 25–30% on positive test

New diagnosis

1.6–2.0 mcg/kg/day

• AbsoluteUntreated thyrotoxicosis, adrenal insufficiency

• Palpitations, tremor (overdose)
• Insomnia, weight loss (overdose)
• Reduced bone density (chronic over-replacement)

• • Crosses placenta minimally — but maternal hypothyroidism causes ↓ fetal IQ.
• • Safe in breastfeeding.

• Iron, calcium, PPIs

  Caution

  ↓ absorption — separate by ≥4 h

• Oestrogen (HRT)

  Caution

  ↑ TBG → may need ↑ dose

• 💡 Take fasting, 30–60 min before breakfast.
• 💡 Iodine supplementation in pregnancy 150 mcg/day.