5th April 2024

Pregnancy and thyroid diseases

Hyperthyroidism in pregnancy

To confirm biochemical hyperthyroidism during pregnancy, the following levels should be checked:

• Serum TSH
• Free thyroxine (FT4)
• Free triiodothyronine (FT3)

Serum TSH-receptor antibodies should be checked immediately once pregnancy is confirmed if there is current or previous history of Grave’s disease.

Risks associated with untreated hyperthyroidism in pregnancy include:

• Miscarriage
• Premature birth
• Low birthweight
• Preeclampsia
• Thyroid storm
• Congestive heart failure

Management can be difficult requiring specialist input. Propylthiouracil is preferred in early pregnancy as it is less likely to cross the placenta than carbimazole, which is associated with congenital abnormalities. Carbimazole is preferred later on in pregnancy due to the risk of severe hepatotoxicity associated with propylthiouracil. Radioactive iodine and traditional block and replace regimens are contraindicated in pregnancy. It is important to check TSH receptor antibody status at 24-28 weeks to assess the risk of foetal/neonatal hyperthyroidism.

Hypothyroidism in pregnancy

Risks associated with untreated hypothyroidism in pregnancy include the following:

• Increased risk of miscarriage
• Anaemia
• Pre-eclampsia
• Placental abruption
• Postpartum haemorrhage
• Stillbirth

Adverse neonatal outcomes include: preterm delivery, low birthweight, neonatal respiratory distress, congenital abnormalities, congenital hypothyroidism and impaired foetal neurocognitive development.

Management is with levothyroxine by maintaining maternal free thyroxine levels in the upper third of normal reference ranges. Thyroxine is safe in pregnancy and breastfeeding. TSH should be checked 4-6 weeks postpartum.

Postpartum thyroid dysfunction

Postpartum thyroiditis (PPT) is a transient autoimmune lymphocytic thyroiditis which can result in thyrotoxicosis, hypothyroidism or thyrotoxicosis followed by hypothyroidism (Table 4). It usually resolves within a year but there is an increased risk of developing permanent hypothyroidism. Anti-thyroid peroxidase antibodies can be found in 90% of people with postpartum thyroiditis.

References:

• National Institute for Health and Care Excellence (UK). Postpartum. Updated May 2021. Available at: https://cks.nice.org.uk/topics/hypothyroidism/management/postpartum/
• Patient UK. Thyroid disease in pregnancy. Last updated 26ᵗʰ June 2016. Available at: https://patient.info/doctor/thyroid-disease-in-pregnancy