The most common cause of an over-active thyroid [hyperthyroidism] is Graves' disease.

Before pregnancy

If hyperthyroidism is not treated, you may have lighter, irregular periods and find it difficult to conceive. After treatment, if you are planning to have a baby you should first have a blood test to check your thyroid condition. If you are not planning to get pregnant then use a contraceptive during and after treatment, as normal fertility can return extremely quickly.

If you are pregnant and have (had) Graves' disease it is important to tell your obstetrician about your medical history. Even if your thyroid function has returned to normal you may still have Graves’ antibodies in your blood and these could affect you and/or your baby during or after your pregnancy. You should also tell the doctor looking after your thyroid, that you are pregnant, as soon as possible. You need to have regular check-ups.

In men, hyperthyroidism can cause a marked reduction in sperm count, resulting in infertility. The sperm count usually returns to normal once the thyroid condition has been treated.

During pregnancy

If you have active hyperthyroidism, you will still need to take antithyroid drugs during your pregnancy. These drugs cross the placenta so the lowest possible dose will be prescribed so that your baby is less likely to be affected.

  • If you are already on Carbimazole (CMZ), you should change to Propylthiouracil (PTU) as soon as possible. PTU is the drug of choice when trying to conceive (preconception) and in the first three months of pregnancy. If PTU is not available CMZ can be used.
  • ‘Block and replace’ therapy (blocking the thyroid from working using CMZ or PTU and then preventing hypothyroidism with levothyroxine tablets) should not be used in pregnancy.
  • Thyroid surgeryis rarely required. If needed it should ideally be performed during the middle three months of pregnancy.
  • Radioactive iodine, another treatment for hyperthyroidism, is never used during pregnancy.

Complications to be alert to if you have (had) hyperthyroidism:

  1. An increased risk of miscarriage in the early stages of pregnancy, if hyperthyroidism is not under control.
  2. A very slight increased risk of the baby developing structural abnormalities, if you are on antithyroid drugs. [So better treat hyperthyroidism with radioactive iodine or surgery before planning for pregnancy]
  3. The baby’s thyroid may become under-active and the baby may develop a goiter, if the dose of antithyroid drugs is very high.
  4. Complications of high blood pressure, poor growth of baby and premature delivery can happen if the hyperthyroidism is not treated.
  5. Temporary hyperthyroidism in babies, which can be treated easily and detected by a simple blood test in mother [Grave’s antibodies]
  6. Very rarely, antithyroid drugs can cause side effects, including agranulocytosis (lowering of the number of white blood cells) and severe liver impairment. 

What you have to do:

  1. When trying to conceive or during pregnancy, do not stop taking antithyroid drugs before speaking to your doctor.
  2. You will require regular thyroid function tests in pregnancy to ensure you are on an appropriate dose.
  3. A simple blood test to measure the thyroid-stimulating hormone receptor antibodies in the mother can help predict whether the baby will be affected in this way. 

After the baby is born

Hyperthyroidism can recur during the first year after the baby is born, so you should  have your blood tested around three months after delivery and at intervals thereafter. If you stopped taking antithyroid drugs during your pregnancy, you should check with your doctor if you notice any symptoms of hyperthyroidism.

Only small amounts of antithyroid drugs cross into breast milk. If you are on antithyroid drugs, you can breast-feed provided the dose is small but check first with your doctor. Antithyroid drugs are best taken in smaller doses over two or three times a day following a feed. If you plan to breast-feed for a long time your baby can have a blood test to check whether its thyroid is being affected.

Mothers with Graves' disease who are not taking antithyroid drugs can safely breast-feed.