Diagnosis

Hyperthyroidism is diagnosed using:

• Medical history and physical exam. During the exam your doctor may try to detect a slight tremor in your fingers when they're extended, overactive reflexes, eye changes and warm, moist skin. Your doctor will also examine your thyroid gland as you swallow to see if it's enlarged, bumpy or tender and check your pulse to see if it's rapid or irregular.

• Blood tests. Blood tests that measure thyroxine and thyroid-stimulating hormone (TSH) can confirm the diagnosis. High levels of thyroxine and low or nonexistent amounts of TSH indicate an overactive thyroid. The amount of TSH is important because it's the hormone that signals your thyroid gland to produce more thyroxine.

  These tests are particularly necessary for older adults, who may not have classic symptoms of hyperthyroidism.

  Thyroid blood tests may give false results if you're taking biotin — a B vitamin supplement that may also be found in multivitamins. Let your doctor know if you are using biotin or a multivitamin with biotin. To ensure an accurate test, stop taking biotin at least 12 hours before blood is taken.

If blood tests indicate hyperthyroidism, your doctor may recommend one of the following tests to help determine why your thyroid is overactive:

• Radioiodine uptake test. For this test, you take a small, oral dose of radioactive iodine (radioiodine) to see how much will collect in your thyroid gland. You'll be checked after four, six or 24 hours — and sometimes after all three time periods — to see how much iodine your thyroid has absorbed.

  A high uptake of radioiodine indicates your thyroid gland is producing too much thyroxine. The most likely cause is either Graves' disease or hyperfunctioning thyroid nodules. If you have hyperthyroidism and your radioiodine uptake is low, this indicates that the thyroxine stored in the gland is leaking into the bloodstream, which may mean you have thyroiditis.

• Thyroid scan. During this test, you'll have a radioactive isotope injected into the vein on the inside of your elbow or sometimes into a vein in your hand. You then lie on a table with your head stretched backward while a special camera produces an image of your thyroid gland on a computer screen. This test shows how iodine collects in your thyroid.
• Thyroid ultrasound. This test uses high-frequency sound waves to produce images of the thyroid. Ultrasound may be better at detecting thyroid nodules than other tests, and there's no exposure to any radiation.