Children are naturally active, but when excess activity is accompanied by a rapid heart rate, trouble breathing, and chronic nervousness, your child may have hyperthyroidism. As in adults, hyperthyroidism is much less common in children than hypothyroidism is. Estimates show that it occurs in 1 of every 5,000 children.

When hyperthyroidism occurs in newborns, it is often a temporary problem caused by a mother having Graves' disease. When a mother has Graves' disease, TSIs are transferred across the placenta to the fetus, causing the baby to produce larger than normal amounts of thyroid hormone. Symptoms of hyperthyroidism include a rapid heart rate, an enlarged thyroid gland, poor weight gain, bulging eyes, and irritability. Until the condition goes away, which may take a few months, your baby may be given an antithyroid drug to control her symptoms. She may also be given a beta-blocker to slow down her heart rate.

Hyperthyroidism in older children resembles that which occurs in adults — nervousness, anxiety, irritability, trouble breathing, and a rapid heart rate are all common symptoms. Some kids may have behavior problems at school and suffer in their academic performance.

But in children, hyperthyroidism may also cause accelerated growth. The effects of this faster growth are usually subtle. For example, over the course of a year or two, the hyperthyroid child's height may climb from the fiftieth percentile to the seventy-fifth. Your child may also have trouble gaining weight and drop to a lower percentile in body weight in spite of a marked increase in appetite. Hyperthyroidism can also delay puberty, or cause it to slow. In girls who have already started their periods, hyperthyroidism can cause their periods to slow or stop completely.

Like in adults, the most common cause of hyperthyroidism in children is Graves' disease, which is discussed in detail in Chapter 9. During pregnancy, maternal antibodies cross the placenta and stimulate the baby's developing thyroid gland, causing it to produce more thyroid hormone.

Treating Graves' disease in children is slightly more complex than it is in adults. Although children have the same treatment options that adults do, some doctors worry that treatment with RAI could raise the risk of thyroid cancer, as well as infertility, later on. For that reason, initial treatment for a child usually involves antithyroid drugs. In many cases, after a few years of treatment, the disease usually goes into remission.

But if the disease is severe and cannot be resolved with drugs, your child may require a more definitive treatment such as surgery or RAI treatments. Concerns about the safety of RAI are based on reports linking external radiation to thyroid cancer. But as of now, there is no hard evidence showing that the low doses involved in RAI actually cause cancer or infertility.

If a child does undergo RAI or surgery, there is a good chance she will become hypothyroid and will require thyroid hormone replacement for the rest of her life.