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Introduction ⬇

A thyroid scan is an imaging test that systematically measures the uptake of radioactive iodine (radioiodine, either 131I or 123I) by the thyroid. Radioiodine is actively transported to the thyroid gland and is incorporated into the production of thyroid hormones. A thyroid scan is usually performed in conjunction with a radioactive iodine uptake (RAIU) test (see Radioactive Iodine Uptake Test). For a complete thyroid workup, in both adults and children, thyroid hormone blood levels are also usually measured. A thyroid ultrasound examination also may be performed.

A thyroid scan is performed to evaluate thyroid size, position, and function. It is used in the differential diagnosis of masses in the neck, base of the tongue, or mediastinum. Thyroid tissue can be found in each of these three locations.

Benign adenomas may appear as nodules of increased uptake of iodine (“hot” nodules), or they may appear as nodules of decreased uptake (“cold” nodules). Malignant areas generally take the form of cold nodules. The most important use of a thyroid scan and uptake test is the functional assessment of these thyroid nodules. Pediatric indications include evaluation of neonatal hypothyroidism or thyrocarcinoma.

Procedure ⬆ ⬇

  1. Administer radioiodine intravenously or have the patient swallow radioactive iodine in a capsule or liquid form. Note: If the patient is to also have an RAIU, a pill or liquid form of radioiodine is given.

  2. Perform thyroid scan 30 minutes after IV injection of radioactive iodine. For oral intake, determine an uptake 4–6 and 24 hours after dosing. Four hours after dosing, the thyroid (neck area) is scanned if 123I is used for both the uptake and the image.

  3. Normal scan time is about 45 minutes.

  4. See Chapter 1 guidelines for safe, effective, informed intratest care.

Clinical Implications ⬆ ⬇

  1. Cancer of the thyroid most often manifests as a nonfunctioning cold nodule, indicated by a focal area of decreased uptake.

  2. Some abnormal results are:

    1. Hyperthyroidism, represented by an area of diffuse increased uptake

    2. Hypothyroidism, represented by an area of diffuse decreased uptake

    3. Graves disease, represented by an area of diffuse increased uptake

    4. Autonomous nodules, represented by focal area of increased uptake

    5. Hashimoto disease (chronic lymphocytic thyroiditis, an autoimmune disease), represented by mottled areas of decreased uptake

  3. Imaging alone cannot definitively determine the diagnosis; uptake information is essential for a definitive diagnosis.

Interventions ⬆ ⬇

Pretest Patient Care

  1. Instruct the patient about a thyroid scan’s purpose, procedure, and special restrictions. Refer to standard NMI pretest precautions.

  2. Because the thyroid gland responds to small amounts of iodine, the patient may be requested to refrain from iodine intake for at least 1 week before the test. Patients should consult with a healthcare provider. Restricted items include the following:

    1. Certain thyroid drugs

    2. Weight-control medicines

    3. Multiple vitamins

    4. Some oral contraceptives

    5. X-ray contrast materials containing iodine

    6. Cough medicine

    7. Iodine-containing foods, especially kelp and other natural foods

  3. Alleviate any fears the patient may have about radionuclide procedures.

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

Clinical Alert

  1. Nuclear medicine thyroid imaging is contraindicated in pregnancy. Thyroid testing in pregnancy is routinely limited to blood testing.

  2. This study should be completed before thyroid-blocking x-ray contrast agents are administered and before thyroid or iodine drugs are given.

  3. Occasionally, tests are performed purposely with iodine or some thyroid drug in the body. In these cases, the healthcare provider is testing the response of the thyroid to these drugs. These stimulation and suppression tests are usually done to determine the nature of a particular nodule and whether the tissue is functioning or nonfunctioning.

Posttest Patient Care

  1. If iodine has been administered, observe the patient for signs and symptoms of allergic reaction.

  2. Explain test outcomes and possible treatment.

  3. Refer to standard NMI posttest precautions.

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

Interfering Factors ⬆ ⬇

  1. Thyroid imaging needs to be completed before x-ray examinations using contrast media (e.g., IV pyelogram, cardiac catheterization, CT with contrast, myelogram) are performed.

  2. Any medication containing iodine should not be given until the nuclear medicine thyroid procedures are concluded. Notify the attending healthcare provider if thyroid studies have been ordered or if there are interfering x-rays or medications.

Reference Values ⬆

Normal

Normal or evenly distributed concentration of radioactive iodine

Normal size, position, shape, site, weight, and function of the thyroid gland

Absence of nodules