Functional thyroid tissue producing an excess of thyroid hormone occurs in a number of clinical conditions. The major causes in humans are:

Graves disease. An autoimmune disease (usually, the most common cause with 50–80% worldwide, although this varies substantially with location- i.e., 47% in Switzerland (Horst et al., 1987) to 90% in the USA (Hamburger et al. 1981)). Thought to be due to varying levels of iodine in the diet. It is eight times more common in females than males and often occurs in young females, around 20 – 40 years of age. Toxic thyroid adenoma (the most common cause in Switzerland, 53%, thought to be atypical due to a low level of dietary iodine in this country)
Toxic multinodular goiterHigh blood levels of thyroid hormones (most accurately termed hyperthyroxinemia) can occur for a number of other reasons:

Inflammation of the thyroid is called thyroiditis. There are several different kinds of thyroiditis including Hashimotos thyroiditis (Hypothyroidism immune-mediated), and subacute thyroiditis (de Quervains). These may be initially associated with secretion of excess thyroid hormone but usually progress to gland dysfunction and, thus, to hormone deficiency and hypothyroidism. Oral consumption of excess thyroid hormone tablets is possible (surreptitious use of thyroid hormone), as is the rare event of eating ground beef or pork contaminated with thyroid tissue, and thus thyroid hormones (termed hamburger thyrotoxicosis or alimentary thyrotoxicosis). Pharmacy compounding errors may also be a cause. Amiodarone, an antiarrhythmic drug, is structurally similar to thyroxine and may cause either under-or overactivity of the thyroid.
On occasion, neck tenderness or a sore throat may become apparent after a few days, if moderate inflammation in the thyroid develops and produces discomfort in the neck or throat area. This is usually transient, and not associated with a fever, etc.It is recommended that breastfeeding be stopped at least six weeks before radioactive iodine treatment and that it not be resumed, although it can be done in future pregnancies. It also shouldnt be done during pregnancy, and pregnancy should be put off until at least 6–12 months after treatment.A common outcome following radioiodine is a swing from hyperthyroidism to the easily treatable hypothyroidism, which occurs in 78% of those treated for Graves thyrotoxicosis and in 40% of those with toxic multinodular goiter or solitary toxic adenoma. Use of higher doses of radioiodine reduces the number of cases of treatment failure, with penalty for higher response to treatment consisting mostly of higher rates of eventual hypothyroidism which requires hormone treatment for life.There is increased sensitivity to radioiodine therapy in thyroids appearing on ultrasound scans as more uniform (hypoechogenic), due to densely packed large cells, with 81% later becoming hypothyroid, compared to just 37% in those with more normal scan appearances (normoechogenic). Thyroid storm
Thyroid storm presents with extreme symptoms of hyperthyroidism.