23rd May 2025

Thyrotoxicosis in pregnancy

Thyrotoxicosis occurs in ~0.2% of pregnancies, where Grave’s disease is the most prevalent clinical thyrotoxic picture. It is considered the 2nd most common endocrine complication to occur in pregnancy after diabetes. Other risk factors for developing gestational hyperthyroidism are hyperemesis gravidarum and trophoblastic neoplasia.

Clinical picture and investigations

• Physiological changes in pregnancy, like increased basal metabolic rate and cardiovascular changes, can mimic hyperthyroidism. The most suggestive symptoms of thyrotoxicosis in pregnancy are failure to gain weight despite adequate intake and tachycardia. Grave’s ophthalmopathy can occur in 1/3 of cases. Symptoms are more severe in the 1st trimester and post-partum periods.
• Diagnosis is confirmed with the lab findings of suppressed TSH and elevated T3 and T4. Normally in pregnancy, T3 and t4 increase in the 1st trimester with low/normal TSH. In the 2nd and 3rd trimester TSH normalises and T3 and T4 are slightly lower.

Complications 

Potential complication due to untreated gestational hyperthyroidism can be either affecting the mother or the foetus:

• Maternal complications
  • Pregnancy-induced hypertension
  • Preterm delivery
  • Congestive heart failure
  • Thyroid storm
  • Miscarriage
  • Abruptio placentae
  • Accidental haemorrhage
• Foetal complications
  • Hyperthyroidism
  • Neonatal hyperthyroidism
  • Intrauterine growth retardation
  • Small-for-gestational age
  • Prematurity
  • Stillbirth
  • Craniosynostosis

Management

• The aim of treatment is alleviation of thyroid symptoms and normalisation of thyroid function. Patients should have their TFTs monitored every 4-8 weeks. The block-and-replace regimen is not used due to the risk of foetal hypothyroidism. ATDs used can be PTU (preferably in the 1st trimester as it can cause child hepatotoxicity). Dosage depends on the TFTs done at 2-4 weeks intervals, aiming to maintain free thyroid hormone in the upper third of the reference range. β-blocker can be used to alleviate the high BMR state, but only prescribed for a few weeks. 
• Iodides and radioiodine are contraindicated in pregnancy. Surgical management is reserved for refractory cases and better done in the 2nd trimester. Breastfeeding is preferred to continue while the mother is on the lowest possible dose of carbimazole. As mentioned before, PTU has greater risk of child hepatotoxicity.