Thyroid Disease and Pregnancy

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 thyoid cannot meet increased demands
  • Plasma binding protein incerases

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 throtoxicosis 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

Causes

  • Most common cause in this age group (fertile women) is Graves' disease
  • Other causes include TMNG, toxic adenoma and thyroiditis

Complications

  • Infertility/ammenorhoea
  • Spontaneous miscarriage
  • Stillbirth
  • Thyroid crisis in labour
  • Transient neonatal thyrotoxicosis

Management

  • Wait and see (supportive management)
    • If hyperemesis, will settle
    • Graves may settle as pregnancy suppresses autoimmunity
    • Check TRAb antibodies - if present alert neonatologist as TRAb antibodies can cross the placenta and cause neonatal transient hyperthyroidism
  • β-blockers if needed
  • LOW DOSE antithyroid drugs - wait as late as possible due to side effects on foetus
    • Propylthiouracil 1st trimester
    • Carbimazole 2/3rd trimester

Post-partum thyroiditis

Aetiology

  • Affects 5% postpartum women (25% in T1DM)
  • Occurs within 6 months of giving birth
    • In the postpartum phase there is exacerbation of all autoimmune diseases

Clinical presentation

  • After delivery, the mother develops transient over-active thyroid, classically at around 6 weeks, and then at around 3 months has an underactive thyroid
  • Will develop small, diffuse, nontender goitre
  • Hypothyroid phase associated with neonatal depression
  • Can persist up to 1 year post partum
  • 25-50% will have persistent hypothyroidism beyond 1 year

Investigations

  • Thyroid function tests
  • Thyroid antibody tests
  • Scintigraphy scan

Management

  • No treatment for hyperthyroid phase, if symptomatic hypothyroid treat with thyroxine
  • Should eventually be able to stop thyroxine but if patient is still on thyroxine after a year it is likely they will need it long term