The term hyperthyroidism refers to any condition in which there is too much thyroid hormone in the body. In otherwords, the thyroid gland is overactive.

SYMPTOMS

Thyroid hormone generally controls the pace of all of the processes in the body.  This pace is called your metabolism. If there is too much thyroid hormone, every function of the body tends to speed up. It is not surprising then that some of the symptoms of hyperthyroidism are nervousness, irritability, increased perspiration, heart racing, hand tremors, anxiety, difficulty sleeping, thinning of your skin, fine brittle hair, and muscular
weakness—especially in the upper arms and thighs. You may have more frequent bowel movements, but diarrhea is uncommon. You may lose weight despite a good appetite and, for women, menstrual flow may lighten and menstrual periods may occur less often.

Hyperthyroidism usually begins slowly. At first, the symptoms may be mistaken for simple nervousness due to stress. If you have been trying to lose weight by dieting, you may be pleased with your success until the hyperthyroidism, which has quickened the weight loss, causes other problems.

In Graves’ disease, which is the most common form of hyperthyroidism, the
eyes may look enlarged because the upper lids are elevated. Sometimes, one or both eyes may bulge. Some patients have swelling of the front of the neck from an enlarged thyroid gland (a goiter).

CAUSES

The most common cause (in more than 70% of people) is overproduction of
thyroid hormone by the entire thyroid gland. This condition is also known as
Graves’ disease. Graves’ disease is caused by antibodies in the blood that
stimulate the thyroid to grow and secrete too much thyroid hormone. This type of hyperthyroidism tends to run in families, and it occurs more often in young women.  Little is known about why specific individuals get this disease.

Another type of hyperthyroidism is characterized by one or more nodules or lumps in the thyroid that may gradually grow and increase their
activity so that the total output of thyroid hormone into the blood is greater than normal. This condition is known as toxic nodular or  multinodular goiter.

Also, people may temporarily have symptoms of
hyperthyroidism if they have a condition called thyroiditis. This condition is
caused by a problem with the immune system or a viral infection that  causes the gland to leak thyroid hormone. It can also be caused by taking too much thyroid hormone in tablet form.