Your thyroid gland produces hormones that control how your body uses energy and help support you and your baby during pregnancy. Thyroid gland dysfunction during pregnancy can lead to too much or little thyroid hormone, which may cause problems for you and your baby. Understanding the thyroid gland’s role in pregnancy will help you better understand why your doctor has recommended supplements or medicines, and when to seek further advice.

What is the thyroid?

The thyroid is a butterfly-shaped gland inside your neck that produces the thyroid hormone. This hormone controls how your body uses energy (your metabolism) and is vital to maintain your health. It is also required to support you and your baby’s health during pregnancy.

How does pregnancy affect thyroid function?

Thyroid hormone levels increase during pregnancy to support you and your baby. While your baby’s own thyroid starts working in week 10 to 12 of pregnancy, it only fully matures in the third trimester. Until then, your baby relies on your supply of thyroid hormone for their brain development.

Good thyroid function is therefore important during pregnancy for both the mother and baby’s health.

What is the difference between an overactive and an underactive thyroid?

During pregnancy, some women may experience thyroid dysfunction where their thyroid becomes:

• An underactive thyroid (hypothyroidism), which can be due to having too little iodine in their body, a pre-existing immune disorder, or previous damage to the thyroid gland; or
• An overactive thyroid (hyperthyroidism), which is usually a result of a condition that stimulates the thyroid gland too much, such as Graves disease.

Thyroid dysfunction affects 2 to 3 in every 100 pregnant women. Either an overactive or underactive thyroid dysfunction should be monitored and treated if necessary.

What are the signs and symptoms of thyroid dysfunction?

Women with an underactive thyroid may report having very low energy levels, feel cold easily, and experience hair loss and/or constipation. However, some women may not notice any symptoms.

Symptoms of an overactive thyroid include weight loss, heat intolerance and high blood pressure.

What tests are used to check thyroid function?

Your doctor can detect unusual thyroid hormone levels with a blood test. Regular screening is recommended for those with a higher risk of thyroid dysfunction, such as pregnant women:

• with symptoms of thyroid dysfunction
• with their own or family history of thyroid disease
• with previous thyroid surgery
• with goitre (swelling of the thyroid)
• with a history of miscarriage or premature delivery
• a history of infertility
• with type 1 diabetes
• who are severely overweight or obese
• who are aged 30 or older

If you feel you fit into one or more of the above categories, ask your doctor or midwife if your thyroid function should be checked.

How is thyroid dysfunction in pregnancy managed?

If you have hypothyroidism, your doctor may prescribe thyroxine tablets before and after conception to optimise your thyroid hormone levels. This treatment is usually accompanied by regular thyroid function tests to ensure your thyroid level is in the recommended range. If left untreated, low levels of thyroid hormone can cause complications in pregnancy, such as premature birth, low birth weight and miscarriage. It may also affect the intellectual development of your baby.

If you have hyperthyroidism, your doctor or specialist may prescribe medication to block thyroid hormone production (antithyroid therapy). The most common medicine used for this is called propylthiouracil. Uncontrolled overactive thyroid during pregnancy has been associated with complications such as a higher fetal heart rate (tachycardia), small gestational size, premature birth or stillbirth.

What is the link between iodine deficiency and thyroid function in pregnancy?

Having too little iodine in your body is a common cause of low thyroid hormone levels. A study found that, on average, Australian women consume 100 micrograms of iodine a day. However, the World Health Organization recommends a daily intake of least 250 micrograms of iodine during pregnancy and breastfeeding.

Australian guidelines recommend that pregnant and breastfeeding women take a supplement containing 150 micrograms of iodine each day (this amount is found in most supplements).

Check with your antenatal health provider (doctor or nurse) if you are concerned about your iodine levels, particularly if you are taking supplements such as pregnancy or pre-natal vitamins and minerals.

Your pharmacist will also be able to guide you on the potential impact of your prescription and over-the-counter medicines and their potential impact on iodine absorption and thyroid function.

Does thyroid function return to normal after the baby is born?

Most pregnant women recover their regular thyroid function following the birth of their baby. However, 1 in 20 women may experience ongoing irregular thyroid activity (postpartum thyroiditis). In some cases, your doctor may continue your thyroid hormone treatment until you have completed your final pregnancy. If you are taking medicines or supplements for thyroid dysfunction, check with your post-natal health provider after your baby is born — be sure to let them know if you are breastfeeding or bottle feeding your baby.