Hyperthyroidism can result due to multiple reasons some of which include the following:

Graves' Disease: Graves' disease is also known as 'Toxic Diffuse Goiter' and it is the most common cause of hyperthyroidism in the United States. Graves' disease is an auto immune disease in which the body's immune system starts producing an antibody called as the 'thyroid stimulating immunoglobulin' or TSI. These antibodies mimic the action of Thyroid Stimulating Hormone (TSH) and cause the thyroid to produce too much of thyroid hormones.

Graves' disease is hereditary in nature and it is up to five times more common in women as compared to men. Some other common factors that can trigger Graves' disease are stress, smoking, and radiation to the neck region.

Thyroid Nodules: Thyroid nodules are also known as adenomas or functioning adenomas or even referred to as 'Hot Nodules'. These are basically lumps in the thyroid gland which are usually noncancerous. When these lumps become overactive, they start producing too much of thyroid hormones leading to the state of hyperthyroidism. When multiple such lumps are overactive in a given person, the state is called Toxic Multinodular Goiter (TMNG).

Thyroiditis: Inflammation of the thyroid gland may occur after a viral illness and this is commonly known as subacute Thyroiditis. Another variant of Thyroiditis which is seen after pregnancy is called Lymphocytic Thyroiditis and this can occur in up to 8% of women after delivery.

In either case, inflammation of the gland leads to excess production of the hormone thus giving rise to hyperthyroidism.

Over-dosage of thyroid hormones: Excessive intake of thyroid hormones (prescribed for treatment of hypothyroidism) can lead to a hyperthyroid state. This especially happens if patients don't follow up regularly with their physicians and continue to take the prescribed dose for hypothyroidism without keeping a check on the hormone levels.

Excessive iodine: Intake of excess iodine may also lead to a hyperthyroid state in people who have an abnormal thyroid gland.

High level of TSH: An abnormally high level of Thyroid Stimulating Hormone (TSH) can lead to excess stimulation of the thyroid gland resulting in high level of thyroid hormones. This can occur due to a tumor of the pituitary gland and is relatively rare.

Drugs: Iodine-containing drugs such as amiodarone or certain other medications like interferon-alpha, lithium, etc. may lead to a hyperthyroid state.