THYROID DISORDER AMONG PEOPLE WITH DOWN'S SYNDROME

by Dr Jennifer Dennis, Medical Adviser, The Down's Syndrome Association

The thyroid gland

The thyroid gland is situated in the neck in front of the windpipe, just below the larynx (voice box). It produces several chemical substances called hormones which circulate round the body in the blood. These hormones, one of which is thyroxine, help to regulate the body's energy level. Thyroxine has a major influence on physical and mental development and on general well-being, because it helps to control the rate of chemical reactions in all the body cells.

Thyroid disorder

If the thyroid gland is overactive (hyperthyroidism) a person may be agitated and jittery, lose weight, and suffer palpitations. If the gland is underactive (hypothyroidism) a person can become tired, overweight, and generally sluggish, with slow physical and mental reactions.

People with Down's syndrome do sometimes have an overactive thyroid, but it is far more common among them for the gland to be underactive. We know that in 15 to 20% of adolescents with Down's syndrome, the thyroid gland is not working properly (i.e. not enough thyroxine is produced to keep the body running at an optimum rate). These people benefit greatly from thyroid replacement therapy, which involves taking one or two tablets of a thyroxine preparation every day.

Diagnosis

It is far more difficult to diagnose thyroid deficiency in children and adults with Down's syndrome than in the general population. Some classic features of poor thyroid function (tiredness, overweight, and general sluggishness) have already been mentioned. In addition the person may feel the cold, have a tendency to constipation, have dry skin, sparse hair, and a rather hoarse voice. Those with a relative or friend with Down's syndrome will recognise that similar features can in any case be quite common in people with the syndrome, so these signs cannot be relied upon to identify those who would benefit from thyroid replacement therapy. The only way we can be sure of recognising those in need of treatment is to carry out regular blood tests.

Testing

It is standard policy in this country for all new-born babies to have their thyroid function tested. A spot of blood from a heel prick is collected on a piece of filter paper and allowed to dry. This is sufficient to allow several tests to be carried out to check for some serious disorders which, if left untreated, are known to cause learning disabilities. Because of this test, children rarely suffer grave long-term effects of thyroid deficiency at birth. These tests are carried out on all babies, including those with Down's syndrome. Parents are only notified if the results are abnormal.

In childhood and in adult life, different doctors follow different schedules and timings for tests and checks. At a minimum it is probably sensible to check all children at least once before their 5th birthday, and thereafter at least every 5 years. If certain warning signs show up in the blood tests, this alerts a doctor to carry out more frequent checks. Some doctors prefer to do the routine tests every year or so.

Thyroid function tests

There are several tests of thyroid function, known collectively as "thyroid function test" (TFTs).

The two most frequently used are a test which measures the level of thyroid hormone (thyroxine, often written as T4) in the blood, and a test which measures the level of a hormone called thyroid-stimulating hormone (TSH).

TSH is produced by a gland underneath the brain and it "drives" the thyroid gland to produce thyroxine. If the thyroid gland is not working well, the TSH levels may be unduly high because more TSH than usual is being required to "drive" it. High TSH levels can therefore be a warning that the thyroid gland, though currently producing enough thyroxine, may become underactive in the future. However TSH levels can be difficult to interpret in children with Down's syndrome. Quite a number of these children seem to produce high TSH levels in early childhood for no obvious reason, and these levels subsequently return to normal. If a child, or an adult, has a high level of TSH in the blood this does not necessarily mean that there is a shortage of thyroxine. It does mean that a doctor will probably want to check the thyroxine levels more often than otherwise. Sometimes different additional tests may be needed.

Apart from these tests, which check how much hormone is circulating in the blood, there is one other standard test which a doctor may use. This is based on one of the causes of thyroid problems. There are several different causes of thyroid underactivity, but the most common among people with Down's syndrome is that they themselves start to produce substances called thyroid antibodies. These antibodies harm a person's own thyroid gland and prevent it producing enough thyroxine. This type of disorder, which also occurs in the general population, is called auto-immune disorder. A blood test can be performed to detect the presence of thyroid antibodies.

At least 30% of adults with Down's syndrome produce these antibodies. About half of them continue to produce enough thyroxine for the body's needs and never need thyroid replacement therapy. However, for the other half the presence of these antibodies in early blood tests can be a useful warning sign that thyroid deficiency may develop in the future. If thyroid antibodies are found in a blood test a doctor may want to check thyroxine levels more often than otherwise.

Treatment

Thyroid dysfunction is much the same in Down's syndrome as in the general population. The same treatment is used. Someone with an underactive thyroid gland can be treated by giving one or two tablets a day of replacement thyroxine. Some people with Down's syndrome respond more slowly to treatment than the general population and advice from a specialist (an endocrinologist) may be needed.

The far less common situation of overactivity can usually be treated with medicines to stop the overproduction of thyroid hormones, but occasionally an operation may be necessary to remove part of the gland.

Summary

The main problem about disturbed thyroid function in people with Down's syndrome is that it can be very difficult to diagnose on clinical grounds (i.e. by a doctor asking questions and carrying out a physical examination), particularly in the early stages. Doctors have to rely on the results of blood tests to make sure they are not missing any cases. If thyroid deficiency is diagnosed the problem then, from a medical point of view, is the same as for anyone in the general population and treatment is both possible and very worthwhile.

Parents and carers

It is very important that parents, carers, and other non-medical professionals, should be aware of the possibility of thyroid underactivity in a person with Down's syndrome, since they may be the first to notice early changes. If a person with Down's syndrome starts to slow down physically and intellectually, and to put on a lot of weight, then consult a doctor about the possibility that the thyroid gland may not be working properly and whether a blood test should be carried out.

However, it is more likely that there will be no immediate worries about thyroid problems. In that case, just check at some convenient moment with the family doctor, child development clinic, or learning disability team, exactly what is the local policy about regular thyroid tests, in order to ensure that the person with Down's syndrome is tested at the appropriate time.