Thyroid disease and pregnancy

Graves’ disease and pregnancy

Hyperthyroidism occurring during pregnancy is almost always the result of Graves’ disease. It is not a common event, however, as autoimmune diseases (see Glossary), of which Graves’ disease is an example, tend to improve of their own accord during pregnancy. Also, women with an overactive thyroid gland are relatively infertile because there is an increase in the number of menstrual cycles during which an egg is not released from the ovaries.

As the thyroid-stimulating antibody responsible for the hyperthyroidism of Graves’ disease crosses the placenta and passes from the blood of the mother to that of the developing child, it too will have an overactive thyroid gland like its mother. Fortunately, the antithyroid drugs also cross the placenta and good control of hyperthyroidism in the mother will ensure that the fetus comes to no harm. Failure to recognise hyperthyroidism or to treat it adequately in a pregnant woman may lead to miscarriage. Overtreatment with antithyroid drugs may lead to goitre development in the fetus. It is important, therefore, that the patient is prescribed the lowest dose of carbimazole possible to restore thyroid hormone levels in the blood to normal. These levels are checked every four to six weeks, in close cooperation with the obstetrician who is caring for her.

The carbimazole is usually stopped four weeks before the expected date of delivery to make sure that there is no possibility of the fetus being hypothyroid at a crucial time in its development. If hyperthyroidism recurs in the mother after the baby is born, and she is breast-feeding, she will be treated with propylthiouracil rather than carbimazole because it is excreted in the breast milk much less and will not therefore affect the baby.

There are some reports from North America that carbimazole is associated with a rare disease in the newborn baby, known as aplasia cutis, in which there is a defect in the skin covering a small part of the scalp. The view in the UK is, however, that the risk has been overestimated, if it is present at all. Most specialists in this country are happy to prescribe carbimazole during pregnancy. Some, however, may prefer to use propylthiouracil and to change from carbimazole before conception, if possible. The dose of propylthiouracil is ten times that of carbimazole, and it is available as 50 milligram tablets only.

Radioactive iodine treatment is never given during pregnancy. Surgery is occasionally advised around week 20 of pregnancy for patients who develop side effects to the drugs or who take them irregularly, thereby putting the fetus at risk.

Hyperthyroidism in the newborn (neonatal thyrotoxicosis)

In most women with Graves’ disease during pregnancy, the thyroid-stimulating antibody disappears or its level in the blood becomes low. In some, however, the level remains high and, as blood from the mother is exchanged with that of the fetus until the moment of birth, these high levels are also present in the blood of the newborn and may cause hyperthyroidism. Although it is possible to predict those babies most likely to develop hyperthyroidism by finding high levels of antibodies in the mother’s blood towards the end of pregnancy, all newborns in the UK have a blood test shortly after birth to check thyroid hormone levels.

Hyperthyroidism in the newborn, if detected at this stage, is easily treated and lasts only two to three weeks until the antibody from the mother is broken down and inactivated. Very occasionally, mothers who have been treated successfully for Graves’ disease in the past continue to produce thyroid-stimulating antibody and their offspring are at risk of developing neonatal hyperthyroidism.

Case history: Rebecca

Rebecca and her husband had been trying to have a second child for three years without success. Rebecca had conceived twice but, unfortunately, on each occasion had miscarried at about ten weeks. She felt and looked well and, although she had lost a few pounds in weight, she put this down to her busy lifestyle of running the home, looking after an active five-year-old son, and working part-time as a secretary. She was a little anxious that her periods, which used to be as regular as clockwork, had become much lighter and, on occasion, were missing.

During her weekly telephone call to her mother, she learned that her cousin in Australia had recently been diagnosed as having an overactive thyroid gland. She consulted her GP and, despite her lack of obvious signs and having neither a goitre nor bulging eyes, the blood test showed the presence of mild hyperthyroidism and this was confirmed as being caused by Graves’ disease at the local hospital. Treatment was started with carbimazole, initially in a dose of 30 milligrams daily and, after five months of treatment, Rebecca was pregnant.

She was reviewed by the endocrinologist every four weeks and, by the middle of her pregnancy, she needed to take only five milligrams of carbimazole every day. The drug was stopped four weeks before the expected date of delivery and she gave birth to a healthy girl whose heel-prick blood test at seven days was normal, with no evidence of thyroid abnormality. Rebecca breast-fed her daughter but, after four months, developed hyperthyroidism, again as a result of Graves’ disease because the thyroid-stimulating antibody was present in her blood. She decided to change to bottle-feeding and her hyperthyroidism was therefore treated with carbimazole as before. Had she opted to continue breast-feeding, propylthiouracil would have been prescribed instead.

Hypothyroidism and pregnancy

Most women with hypothyroidism are already taking thyroxine when they become pregnant. Although mild hypothyroidism is unlikely to affect fertility, women with severe thyroid deficiency of prolonged duration are unlikely to become pregnant and, if they conceive, to maintain their pregnancy.

The dose of thyroxine may need to be increased in pregnancy. Recent research shows that the increase is most important to the fetus in early pregnancy. As soon as you become pregnant visit your doctor who will probably increase your thyroxine dose by 25 micrograms and carry out a blood test. You will be tested every two months or so during pregnancy and the average extra dose of thyroxine needed will be 50 micrograms daily. You can return to the dose you were taking before pregnancy after your baby has been born.

Although the thyroid gland of the fetus develops independently of the mother and makes its own thyroid hormones, a recent study in the USA has shown that unrecognised or inadequately treated hypothyroidism in the mother may cause a slight reduction in the IQ of the child. Your baby will not be at risk if you forget the occasional dose, but if you make a habit of not taking it, not only will you face a greater risk of miscarriage, but your baby may not be as intelligent as he or she might have been.

It would be sensible for those taking thyroxine or those who have a family history of thyroid disease to check that their thyroid blood tests are normal when planning a pregnancy, and therefore before conception.

Hypothyroidism in the newborn (congenital hypothyroidism)

One in about 3,500 newborn babies has an underactive thyroid gland as a result of failure of the gland to develop normally. In the past, the problem was not recognised until the child was several weeks old, by which time he or she would have been likely to develop permanent mental and physical handicap – the condition known then as cretinism. Today, however, all newborn babies are screened by a blood test for hypothyroidism between five and seven days after they are born. Any affected children are given prompt treatment which ensures that they develop normally.

Treatment is usually for life, but in a few babies the hypothyroidism is temporary as a result of being born to a mother with an underactive thyroid gland; in these women there are blocking antibodies that cross the placenta and have the opposite effect of the stimulating antibodies of Graves’ disease and neonatal thyrotoxicosis.

Thyroid disease after pregnancy

Although the hyperthyroidism of Graves’ disease tends to get better on its own during pregnancy, it often returns in a severe form within a few months of delivery. There is, however, another form of hyperthyroidism that may develop in the first year after childbirth, almost always in patients who have an underlying autoimmune thyroid disease such as Hashimoto’s thyroiditis, which may not have been recognised previously.

The hyperthyroidism is mild and lasts only a few weeks; if treatment is necessary, only a beta blocker is taken. This phase may be followed by an equally transient episode of mild hypothyroidism not requiring treatment, and then usually by full recovery. A similar pattern may occur in future pregnancies and many patients ultimately develop a permanently underactive thyroid.

To distinguish between what is known as postpartum thyroiditis (see Glossary), not requiring treatment, and Graves’ disease, which requires treatment, two measurements may be necessary. One is the concentration of thyroid-stimulating antibody in the blood, as this is usually present in Graves’ disease. The other is the ability of the thyroid gland to concentrate radioactive iodine or technetium, as this is lacking in postpartum thyroiditis.

Postpartum thyroiditis affects about five per cent of women but most patients do not complain of symptoms. There does not appear to be any association between the thyroid blood test abnormalities and postnatal depression.

Case history: Flora

Flora Stewart was 25 and happily married to her lawyer husband, William, and they had had their first child, Jane, five months earlier. Their relationship began to deteriorate when Flora became weepy and short-tempered, snapping at William for no good reason. She was also sleeping badly and William noticed that Flora’s hands sometimes trembled.

However, they both put all this down to hormonal changes following her pregnancy and the birth of their baby, and assumed that before long everything would be back to normal. When Flora began to complain of palpitations, William persuaded her to visit their GP.

The doctor thought that Flora might have an overactive thyroid gland and his suspicions were confirmed by a blood test.

On hearing the news Flora was concerned because her mother had suffered from Graves’ disease when she was in her 30s and her eyes were still very prominent 20 years later, even though the hyperthyroidism had been cured. In order to relieve some of Flora’s symptoms her GP prescribed a long-acting form of propranolol (Inderal LA) 80 milligrams, to be taken once daily and he suggested that Flora should see a specialist at the local hospital. By the time her appointment came round four weeks later, Flora felt much better and a repeat blood sample showed that her thyroid gland had become very slightly underactive.

The diagnosis was not that of Graves’ disease, but of postpartum thyroiditis, and Flora was reassured that she would not get bulging eyes like her mother. The propranolol was stopped, and another blood test two months later was entirely normal.

Flora now knows that she may get the symptoms of postpartum thyroiditis after further pregnancies, and that she has an increased chance of developing a permanently underactive thyroid gland at some stage in the future.

However, her GP will do a thyroid blood test every year to make sure that it is detected before she can develop severe symptoms.

KEY POINTS

• If you are planning a baby, tell your doctor as you may need to take a different drug from your usual one

• Your doctor will keep a close watch on you during pregnancy, but your treatment will not harm your developing baby

• Some women will develop mild thyroid disease after having a baby, but this is easily treated. If you are experiencing similar symptoms to those described in Flora’s story, it is worth asking your GP whether this could be the cause

• Although your child may be born with hypothyroidism or hyperthyroidism, if you suffer from either condition, like all newborns he or she will be given a routine test shortly after birth and treated if necessary