How thyroid problems can affect pregnancy

In conjunction with World Thyroid Day (May 25), we take a look at thyroid problems that may occur during pregnancy, and how this affects the developing foetus.

Thyroid diseases may be broadly divided into two categories:

·Hormonal problems, which may be due to an excess of thyroid hormone production (hyperthyroidism) or the opposite, which is insufficient thyroid hormone production (hypothyroidism).

·An enlarged thyroid gland (goitre) or nodules in the gland, which may be cancerous or non-cancerous.

Generally, only hormonal problems affect pregnancy. Optimal levels of thyroid hormone are essential to health - from foetal life to adulthood. This review will therefore concentrate only on hyperthyroidism and hypothyroidism.

Problems with conception

Both hyperthyroidism and hypothyroidism affect the levels of female hormones, leading to menstrual problems and infertility. In addition, women with hypothyroidism may have raised prolactin levels (the hormone which promotes breast milk production, so high levels simulate breastfeeding), further diminishing fertility.

Diagnosing hypothyroidism in pregnancy is a problem because it is usually "silent". The women at risk would be those with a family history of thyroid disease. However since anyone can develop hypothyroidism, and since it may be harmful to the pregnancy and foetus, all pregnant women and those intending to be pregnant should ideally be screened.

However, this is not universally accepted. In the US, this is the norm, but not in the UK. Other countries go a step further and screen all women over a certain age whether or not they intend to have a child.

The reason for this controversy is because studies have not proven conclusively that population wide screening at the expense of the government will lead to an improvement in pregnancy and foetal outcomes. But if cost is not a problem, I would suggest that pregnant women or those contemplating pregnancy request for a simple blood test to screen for hypothyroidism.

Hyperthyroidism is highly symptomatic (severe weight loss, anxiety, a rapid heart rate, tremors, etc), so screening is usually not necessary.

The risks of hyperthyroidism (an overactive gland)

For the mother:

·PET (severe hypertension in pregnancy) may occur in the mother.

·Increased risk of miscarriage.

·Increased risk of premature delivery.

For the foetus:

·Foetal malformation.

·Low birth weight.

·Rarely, the baby may be born with hyperthyroidism (Neonatal Graves' Disease) if the mother has this condition (but from my experience this is extremely rare in our population).

The risks of hypothyroidism (an underactive gland)

For the mother:

·PET (severe hypertension in pregnancy).

·Increased risk of bleeding.

·Separation of the placenta from the lining of the womb, causing the foetus to die.

·Increased risk of miscarriage.

·Increased risk of premature delivery.

·Increased need for delivery by Caesarean section.

For the foetus:

·In the first 10 weeks of life, the foetus is completely dependent on the mother for its supply of thyroxine, the hormone normally secreted by the thyroid gland. Inadequate thyroxine at this stage will affect the development of the brain, and some studies suggest that IQ in later life may be impaired. However this has not been corroborated in all studies. In any case, early and adequate thyroxine replacement is prudent.

·The risk of the foetus dying in the womb is increased up to four times.

·The risk of dying within the first month after delivery is also increased.

·Hypothyroidism may also occur spontaneously in the newborn babies of normal mothers. Cord blood screening for hypothyroidism in newborn babies is now standard procedure in most hospitals.

Treatment during pregnancy

For hyperthyroidism, drugs which block the action of thyroid hormones are used. Examples would be carbimazole, methimazole and propylthiouracil.

In theory, propylthiouracil is safer, but in practice, all the drugs work well. The standard practice now is to use propylthiouracil during the first three months of pregnancy and then switch to the other drugs for the duration of the pregnancy.

I must stress again that this is a theoretical consideration. In countries where propylthiouracil is not available (at one stage it was not available in Malaysia either), no problems seem to have arisen.

It is important not to overtreat, because the excess drug may cross the placenta and suppress the foetal thyroid gland too.

Rarely, if the mother's thyroid gland is very large, an operation may be recommended.

Hypothyroidism in pregnancy is a semi-emergency because delayed replacement with thyroxine, especially during early pregnancy, may lead to underdevelopment of the foetal brain. Thyroxine must be started as soon as the diagnosis is made.

Patients with known hypothyroidism and already on thyroxine are advised to increase their dose of thyroxine by 30-50 per cent as soon as they realise they are pregnant. The higher dose should be continued for the duration of the pregnancy.

As for breastfeeding, there is no reason why women with thyroid disease cannot breastfeed.

By the time they deliver, most mothers with hyperthyroidism should be well-controlled and will be on low dose medication. It is safe to breastfeed while taking low dose medication.

Mothers with hypothyroidism who wish to breastfeed their child should continue on the higher dose of thyroxine (the dose used during pregnancy) till the baby is weaned off the breast.

Postpartum thyroiditis

Another interesting condition associated with pregnancy is postpartum thyroiditis. By definition, this occurs within six to twelve months after delivery.

Like most thyroid diseases, it is an "autoimmune" disease, which means that the body produces "toxic" antibodies directed against itself, in this case against the thyroid gland. The gland may first be stimulated, then damaged, and eventually destroyed.

In 30 per cent of cases, permanent hypothyroidism is the result.

The usual presentation is the onset of hypothyroidism, within weeks or months of delivery. It is easy to miss because patients may just complain of lethargy and muscle aches, symptoms not uncommon after delivery. Rarely, patients may transiently present with hyperthyroidism.

In a nutshell

·Thyroid problems (either hyper or hypothyroidism) may occur during pregnancy.

·This may affect the pregnancy, the mother and the foetus.

·Screening is recommended. A simple blood test will suffice.

·Early and adequate treatment of hypothyroidism during pregnancy is important to protect the foetus.

·If pregnancy occurs in a woman with known hypothyroidism, she should immediately increase the dose of thyroxine by 30-50 per cent.

·Thyroid disease may also occur after delivery (postpartum thyroiditis).