Diagnosing hyperthyroidism can be difficult because symptoms associated with the disorder often mimic those of other conditions. Diagnosis typically begins with the physician reviewing the patient's medical history and performing a physical examination.

The patient will be asked to swallow while the physician examines the thyroid gland for changes. The physician also looks for signs such as tremor in the fingers when they are extended, overactive reflexes, changes in the eyes and skin that is warm and moist. Changes in certain vital signs (pulse, rate of breathing) may indicate an increased heart rate. Blood pressure may be high, particularly systolic pressure (the top number in a blood pressure reading).

Thyroid blood tests also are used to diagnose hyperthyroidism. In most cases, levels of thyroid-stimulating hormone (TSH) are low while measures of thyroxine (T4) and triiodothyronine (T3) are high. Sometimes, the TSH can be low but the T4 and T3 are normal.

When blood testing reveals hyperthyroidism, other tests will be performed to pinpoint the cause of the overactive thyroid. During a radioactive iodine uptake test, the patient takes an oral dose of radioactive iodine. Because the thyroid uses iodine to make hormones, the iodine collects in the thyroid gland.

Patients are tested three and/or 24 hours after taking the iodine to see how much of the substance has been absorbed into the thyroid. If too much iodine is present, the thyroid gland likely is producing too much thyroxine, which indicates Graves' disease or hyperfunctioning nodules. If too little iodine is present, the patient may have thyroiditis.

Thyroid scans also can help determine the source of hyperthyroidism. A radioisotope is injected into the patient, who then lies on a table with the chin up and the neck extended backward. A special camera captures images of the thyroid on a computer screen.