General effects of thyroid disease and pregnancy

• Hypo- and hyperthyroidism causes anovulatory cycles - reduced fertility
• Maternal thyroxine important for neonatal development (especially CNS)
• Increased demand on thyroid during pregnancy
  • Increase in size
  • Increased T4 production just to maintain normal concentration
  • Patients who are already on thyroxine will have a relative thyroid deficiency as thyroid cannot meet increased demands
• Plasma binding protein increases

Hypothyroidism in pregnancy

Pre-existing hypothyroidism

• Unable to compensate for increased demand
• Increase thyroxine dose by 25mcg as soon as pregnancy suspected
• Chech TFTs monthly for first 20 weeks then 2 monthly until term
• The average dose increase is by 50% by 20 weeks
• Aim for TSH <3 mU/l

Complications of untreated hypothyroidism in pregnancy

• Increased abortion, preeclampsia, abruption, postpartum haemorrhage, preterm labour
• Impacts on foetal neurophysical development - average of 7 IQ points less in children of untreated hypothyroid mothers vs normal mothers

Abnormal thyroid tests in pregnancy

• Excess hCG effect biochemically mimics hyperthyroidism
  • hCG increases thyroxine and therefore suppresses TSH
  • Both hCG and hyperthyroidism result in high free T4 and low TSH
• In hyperemesis gravidarum, patients will have high hCG and 50% have low TSH (+/- increased fT4)
• To distinguish gestational hCG-associated thyrotoxicosis from hyperthyroidism:
  • Hyperemesis gravidarim - ↑hCG, ↓TSH
  • Not TRab antibody positive
  • Resolves by 20 weeks gestation
  • Only treat if no improvement > 20 weeks

Hyperthyroidism in pregnancy