30th June 2023

Thyroid disease

Thyroid binding globulin, total T4 and T3 are all increased in pregnancy. Thyroid stimulating hormone (TSH) levels also increase in the first trimester before settling again as pregnancy progresses. Beta-hCG is similar in structure to TSH and can therefore lead to transient hyperthyroidism.

Free T3 and T4 are reduced with increased maternal iodine demands.

Due to these changes, the normal range for TSH, T3 and T4 levels are different for each trimester.

Hyperthyroid

Graves’ disease is the leading cause of hyperthyroidism seen in pregnant women, with the majority diagnosed pre-pregnancy. Carbimazole is the first line treatment outside pregnancy, however, there is a theoretical risk of aplasia cutis if used in the first trimester.

If hyperthyroidism is diagnosed in the first trimester, propylthiouracil is considered first line. Later in pregnancy either propylthiouracil or carbimazole can be used. If a woman conceives whilst taking maintenance dose carbimazole, the consensus opinion is to remain on the lowest dose possible to keep levels stable.

Graves’ is an autoimmune condition and therefore is likely to improve in pregnancy, with a potential flare postpartum. Thyrotoxicosis is rare but dangerous in pregnancy and can lead to miscarriage, preterm birth, fetal or neonatal thyrotoxicosis, and thyroid storm around delivery.

Hypothyroid

Hypothyroidism is common in women of childbearing age and can affect up to 1% of pregnancies in the UK. As in the case of hyperthyroidism, most cases will have been diagnosed pre-pregnancy and will already be on replacement therapy.

About a quarter of women will require an increase in dosing but this should be confirmed by checking for TSH and low free T4.

Untreated hypothyroidism can lead to an increase in miscarriage, low birth weights, anaemia, and pre-eclampsia. There is also a link to developmental delay in infancy.