Hyperthyroidism can have several underlying causes. Most common cause is so-called Graves' disease, when the entire thyroid gland is producing excess thyroid hormone. In rare cases, only one nodule of the thyroid gland has gone faulty and is secreting too much hormone. Such nodules are known as "hot" nodules. Inflammation of the thyroid, condition known as thyroiditis is also a cause of hyperthyroidism.
This disease, sometimes referred to as diffuse toxic goiter (goiter is term for swelling of the neck caused by enlarged thyroid gland), is characterized by enlarged thyroid gland that secretes much more thyroid hormones than is necessary. Typically, enlarged thyroid gland (as they come in pairs one or both can be enlarged) secretes less hormone than is needed, so this is a specificity of Graves' disease. Symptoms of Graves' disease include hyperthyroidism, thickening of the skin on the legs below he knees and inflamed and swollen tissue around the eyes.
Most patients do not express obvious problems with the eyes except slight irritation, but in more severe cases (roughly 5% of patients), eyes could be bulging from the pressure of the inflamed tissue and inflammation can be so severe that it can induce blurry or double vision. If left unchecked, such symptoms can lead to permanent eye damage and blindness. Once the hyperthyroidism has been put under control, condition of the eyes gradually improves.
Characteristic features of Graves' disease
Women are far more affected by Graves' disease than men (ratio is about 8:1) and is most common between the ages of 30 and 40, while patients over the age of 50 are rare. It seems to be hereditary as it commonly runs in families, but exact cause (or reason) for this is not known.
Other causes of hyperthyroidism
There are cases in which just one nodule, which is typically a benign tumor of the gland, is secreting the entire excess hormone. This condition is known as "toxic nodular goiter". Thyroiditis, usually caused by infection, can trigger excretion of surplus thyroid hormones and which are normally stored within the gland. Inflammation of the thyroid gland, not caused by an infection, can frequently occur in women few months after delivering a baby.
Excess intake of thyroid hormone for medical purposes can also cause hyperthyroidism. This is common in patients who take form of thyroid hormone known as T3, which is usually present in very minute quantities in organism. There are other forms of hyperthyroidism, but these are very rare.