Hyper thyroidism often called overactive thyroid, is a condition in which the thyroid gland produces and secretes excessive amounts of the free-triiodothyronine (T3) and/or thyroxine (T4).  Hyperthyroidism is one cause of thyrotoxicosis—this is hypermetabolic clinical state which occurs when there are elevated serum levels of T3 and/or T4.

Causes
Grave’s disease is the most common cause of hyperthyroidism, an autoimmune disease thought to be due to varying levels of iodine in the diet. Some people develop thyrotoxicosis due to over inflammed thyroid gland (thyroiditis), or thyroid nodules. Thyrotoxicosis can also occur after taking too much thyroid hormone in the form of supplements, such as levothyroxine (a phenomenon known as exogenous thyrotoxicosis or alimentary thyrotoxicosis.)

Others are postpartum thyroiditis, which occurs in some women following delivery. Struma Ovarii- a kind of ovarian tissue disorder that contains thyroid tissue and secrets thyroid hormones. Pituitary disorder leading to hypersecretion of TSH

Symptoms
Thyrotoxicosis or Hyperthyroidism may be asymptomatic, but when accompanied with symptoms, these are due to an excess of thyroid hormone. Thyroid hormone is important at the cellular level, affecting nearly every type of tissue in the body. Thyroid hormone functions as a controller of the pace of all of the processes in the body. This pace is called the metabolic rate).

If there is too much thyroid hormone, every function of the body tends to speed up. Therefore, some of the symptoms of hyperthyroidism may be nervousness, irritability, increased perspiration, heart racing, hand tremors, anxiety, difficulty sleeping, thinning of the skin, fine brittle hair, and muscular weakness. More frequent bowel movements may occur, but diarrhea is uncommon. Weight loss, sometimes significant, may occur despite a good appetite, vomiting may occur, and, for women, menstrual flow may lighten and menstrual periods may be irregular.

In Graves’s disease, which is the most common form or cause of hyperthyroidism, the eyes may look enlarged because the eye muscles swell and push the eye forward.  Sometimes, one or both eyes may bulge. Some have swelling of the front of the neck from an enlarged thyroid gland (goitre).

Diagnosis
The diagnosis of hyperthyroidism is done when blood tests show a decreased thyroid-stimulating hormone (TSH) level and elevated T4 and T3 levels.  A radioactive iodine uptake test and thyroid scan together can be done to determine the cause of hyperthyroidism. Subclinical hyperthyroidism is a milder form that is seen with low or undetectable serum TSH level, but with a normal serum free thyroxine level.