🔑 Key Learning

  • Pregnancy increases thyroid binding globulin (TBG), leading to raised total T4, but free T4 remains unchanged.
  • TSH should be checked every trimester and 8 weeks postpartum.
  • Levothyroxine is safe in pregnancy and breastfeeding.
  • Women with pre-existing hypothyroidism should increase their dose by 25–50 mcg as soon as pregnancy is confirmed.

🧬 Physiology

  • In pregnancy, thyroid binding globulin (TBG) increases due to oestrogen.
  • This causes a rise in total T4.
  • Free T4 levels remain unchanged (the biologically active form).
  • hCG can weakly stimulate TSH receptors, leading to transient reduction in TSH in early pregnancy.

🧪 Monitoring

  • Check TSH levels:
    • Once per trimester
    • 8 weeks postpartum
  • Target: TSH should remain within trimester-specific reference ranges.

💊 Levothyroxine Use

  • Safe in pregnancy and breastfeeding.
  • Pregnancy increases thyroxine requirements — usually due to higher TBG and metabolic demands.
  • As soon as pregnancy is confirmed in a woman with stable hypothyroidism:
    • Increase levothyroxine by 25–50 mcg daily (e.g. 125 mcg → 175 mcg).
    • This should be done immediately, without waiting for TFT results.
  • If unsure, seek specialist advice promptly — but do not delay dose adjustment.

📝 Exam Clues & Clinchers

  • Rising TSH in a woman already on levothyroxine = needs dose increase.
  • Safe management = continue levothyroxine, increase dose as above, regular TSH checks.