Tests and diagnosis

By Mayo Clinic Staff

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.
  • Blood tests. A diagnosis can be confirmed with blood tests that measure the levels of thyroxine and TSH in your blood. 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.

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

  • Radioactive iodine uptake test. For this test, you take a small, oral dose of radioactive iodine (radioiodine). Over time, the iodine collects in your thyroid gland because your thyroid uses iodine to manufacture hormones. You'll be checked after two, six or 24 hours — and sometimes after all three time periods — to determine how much iodine your thyroid gland 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 nodules. If you have hyperthyroidism and your radioiodine uptake is low, you may have thyroiditis.

    Be sure to tell your doctor if you have had a recent X-ray or a computerized tomography scan in which you had contrast material was injected. The results of your radioiodine test may be influenced by these procedures.

    Knowing what's causing your hyperthyroidism can help your doctor plan the appropriate treatment. A radioactive iodine uptake test isn't uncomfortable, but it does expose you to a small amount of radiation.

  • 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 on a computer screen.

    The time needed for the procedure may vary, depending on how long it takes the isotope to reach your thyroid gland. You may have some neck discomfort with this test, and you'll be exposed to a small amount of radiation.

    Sometimes you may have a thyroid scan as part of a radioactive iodine uptake test. In that case, orally administered radioactive iodine is used to image your thyroid gland.

Nov. 20, 2012

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