I met a patient today who was pregnant and had hypothyroidism, so I read up a little about how pregnancy affects the thyroid and vice versa.

What normally happens


The way I think about it is that as the baby grows, the mum’s liver is going to have to work harder, and so it starts producing more globulins that can bind to thyroid hormone. These globulins then start mopping up free thyroid hormone in the blood, and the thyroid has to start making more to restore the right levels.

Thus several things happen:

  • Levels of total T3 and T4 increase
  • Levels of free hormone remain roughly the same
  • Levels of TSH remain the same

Next, it might be that the body notices the thyroid starting to grow, and gets wary – so it might start producing antibodies against various components of thyroid tissue. Probably the most important are anti-thyroid peroxidase antibodies. This happens in 1/10 women, usually around 14 weeks.

These antibodies can lead to:

  • loss of pregnancy
  • gestational thyroid disease
  • post partum thyroiditis

Too low?

Around 2.5% of pregnant women can be classed as hypothyroid, however only about 0.3% are actually affected by it.

How I remember causes of hypothyroidism in pregnancy: Acid will rip your thyroid to shreds

  • Autoimmune
  • Congenital
  • Iodine deficiency
  • Drugs – lithium, amiodarone
  • Radioactive contrast
  • Infiltrative disease
  • Pituitary or hypothalamic dysfunction

How I remember presentation of hypothyroidism in pregnancy:

I threw a dry yellow ball at Mrs Hypothyroid, but her reflexes were so slow that it smacked her in the eye and knocked all her hair out. She then started shivering and her neck started to swell, and then she said in a really hoarse voice that she couldn’t sleep. Then she tried to go to the loo but couldn’t stand up and started bleeding all over the floor.

  • dry skin
  • yellowing around the eyes
  • diminished tendon reflexes
  • thinning of hair
  • cold intolerance
  • goitre
  • hoarse voice
  • sleep disturbance
  • constipation
  • weakness
  • anaemia

The way I manage complications of hypothyroidism in pregnancy:

I asked Mrs Hypothyroid how she was feeling, she said it was complicated. Her heart hurts and she keeps craving salt. Then all of a sudden her abdomen started to swell and she began sweating and shaking. She said she was hearing voices and her shins began to swell up. Then she fell asleep. 

  • congestive heart failure
  • hyponatraemia
  • megacolon
  • adrenal crisis
  • psychosis
  • myxoedemal coma

Management is pretty simple: supplement the thyroid hormone

  • if mum was on thyroxine before pregnancy, it will need a 30% boost during pregnancy
  • after birth, return to normal dose, and check TSH 6-8 weeks later
  • it’s quite likely that women with auto-antibodies to the thyroid in pregnancy may develop post-partum thyroid dysfunction, so they need monitoring!

Prognosis tends to be good, although recent research says that maternal hypothyroidism can lead to neurodevelopmental problems in the baby, even if the baby is euthyroid, as the maternal thyroid hormone helps govern neuronal development until 13 weeks gestation.

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