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Introduction ⬇

The RAIU test evaluates the function of the thyroid gland by measuring the ability of the gland to concentrate and retain iodine. When radioactive iodine is administered, it is rapidly absorbed into the bloodstream. This test measures the rate of accumulation, incorporation, and release of iodine by the thyroid. The rate of absorption of the radioiodine, which is determined by the increase in radioactivity of the thyroid gland, is a measure of the ability of the thyroid to concentrate iodine from blood plasma. The radioactive isotopes of iodine used are 131I and 123I.

This test is indicated in the evaluation of hypothyroidism, hyperthyroidism, thyroiditis, goiter, and pituitary failure and for posttreatment evaluation. The patient who is a candidate for this test may have a lumpy or swollen neck or complain of pain in the neck; the patient may be jittery and ultrasensitive to heat or sluggish and ultrasensitive to cold. The test is more useful in the diagnosis of hyperthyroidism than hypothyroidism.

Procedure ⬆ ⬇

  1. A fasting state is preferred. A complete history and listing of all medications is a must for this test. This history should include over-the-counter medications, as well as herbal supplements, vitamins, and patient dietary habits.

  2. Administer a liquid form or a tasteless capsule of radioactive iodine orally.

  3. Measure the amount of radioactivity by an uptake calculation of the thyroid gland 4 or 6 hours later, and again 24 hours later by placing a gamma probe over the thyroid gland in the neck. Explain to the patient that there is no pain or discomfort involved.

  4. Have the patient return to the laboratory at the designated time because the exact time of measurement is crucial in determining the uptake.

Clinical Implications ⬆ ⬇

  1. Increased uptake suggests hyperthyroidism but is not diagnostic for it.

  2. Decreased uptake may be caused by hypothyroidism but is not diagnostic for it.

    1. If the administered iodine is not absorbed, as in severe diarrhea or intestinal malabsorption syndromes, the uptake may be low even though the gland is functioning normally.

    2. Rapid diuresis during the test period may deplete the supply of iodine, causing an apparently low percentage of iodine uptake.

    3. With kidney failure, the uptake may be high even though the gland is functioning normally.

Clinical Alert

  1. This test is contraindicated in pregnant or lactating women, in children, in infants, and in persons with iodine allergies.

  2. Whenever possible, this test should be performed before any other radionuclide procedures are done, before any iodine medications are given, and before any x-rays using iodine contrast media are taken.

Interventions ⬆ ⬇

Pretest Patient Care

  1. Explain test purpose and procedure; the test takes 24 hours to complete. Assess and record pertinent dietary and medication history.

  2. Advise that iodine intake is restricted for at least 1 week before testing.

  3. Refer to standard NMI pretest precautions.

  4. Follow guidelines in Chapter 1 for safe, effective, informed pretest care.

Posttest Patient Care

  1. Explain test outcomes and possible treatment.

  2. Refer to standard NMI posttest precautions.

  3. Review test results; report and record findings. Modify the nursing care plan as needed. Counsel the patient appropriately.

  4. Follow guidelines in Chapter 1 for safe, effective, informed posttest care.

Interfering Factors ⬆ ⬇

  1. The chemicals, drugs, and foods that interfere with the test by lowering the uptake are:

    1. Iodized food and iodine-containing drugs such as Lugol solution, expectorants, cough medications, saturated solutions of potassium iodide, and vitamin preparations that contain minerals: The duration of the effects of these substances in the body is 1–3 weeks.

    2. X-ray contrast media such as iodopyracet, sodium diatrizoate, poppyseed oil, ethiodized oil, iophendylate, and iopanoic acid: The duration of the effects of these substances is 1 week to 1 year or more; consult with the nuclear medicine laboratory for specific times.

    3. Antithyroid drugs such as propylthiouracil and related compounds: The duration of the effects of these drugs may last 2–10 days.

    4. Thyroid medications such as liothyronine sodium, desiccated thyroid, thyroxine (levothyroxine sodium): The duration of the effects of these medications is 1–2 weeks.

    5. Miscellaneous drugs such as thiocyanate, perchlorate, nitrates, sulfonamides, tolbutamide, corticosteroids, para-aminosalicylate, isoniazid, phenylbutazone, thiopental, antihistamines, adrenocorticotropic hormone, aminosalicylic acid, cobalt, and warfarin sodium anticoagulants: Consult with the nuclear medicine department for duration of effects of these drugs as they vary widely.

  2. The compounds and conditions that interfere by enhancing the uptake are:

    1. Thyroid-stimulating hormone (TSH) (thyrotropin)

    2. Pregnancy

    3. Cirrhosis

    4. Barbiturates

    5. Lithium carbonate

    6. Phenothiazines (duration, 1 week)

    7. Iodine-deficient diet

    8. Kidney disease

Reference Values ⬆

Normal

Absorption (uptake) by the thyroid gland:

  • 3%–16% after 6 hours

  • 8%–25% after 24 hours

Values are laboratory dependent.