Introduction

The thyroid uptake and scan is a radiologic diagnostic tool used to determine thyroid function and pathologies. This procedure works on the principle of the unstable nuclide of the atom, which tries to attain stability by releasing alpha, beta, and gamma rays. Clinicians use these rays for diagnostic and treatment purposes.

The test uses a radioactive tracer, a protein, or a molecule attached to radioactive material. The radioactive tracer is administered to the patient, and a probe measures the amount of iodine uptake by the thyroid gland. The thyroid scan checks for the even spread of the tracer in the gland. More than 20 radioactive tracers exist, of which 2 isotopes of iodine I-123 and I-131 and 99m-technetium pertechnetate are the usual agents used in this test. I-131 has a longer half-life than I-123, so I-123 is used more frequently than I-131 due to less radiation exposure to the body. 99m-Technetium pertechnetate is an analog of iodine, so it gets transported to the thyroid gland similarly to iodine. 111In-pentoxide, thallium-201 (201T1), 99mTc-sestamibi, and 99mTc-tetrofosmin are some of the less commonly used radioactive tracers in this test.[1] This test is different than radioactive iodine therapy, which treats thyroid cancers. 

Procedures

Clinicians usually perform the thyroid uptake and scan in the outpatient setting. The patient should be prepared well for this, as certain foods and medications interfere with radiotracer uptake in the thyroid gland. The patient can be allergic to the radiotracer, so careful evaluation is necessary. If a patient is taking antithyroid medications such as methimazole or propylthiouracil, they should receive instructions to hold these medications for at least 5 days. Before the test, prior studies of the thyroid gland and baseline TSH and free T4 levels are necessary. In thyroid cancer patients, thyroglobulin and thyroglobulin antibodies may also need to be checked.

Radiotracer is given to the patient intravenously or orally. The timing of radiotracer administration before the scan varies according to the route. The patients are asked to take an oral radiotracer 24 hours before the scan or be injected with a radiotracer 30 minutes before the scan. Sodium iodide symporters located on the thyroid gland cells are responsible for uptaking radioactive iodine. The patient then lies on the movable examination table, and the gamma camera takes serial images of the thyroid gland from 3 different angles. The patient should lie still while capturing images. The clinical history, thyroid examination, and thyroid function tests all merit consideration while interpreting the results of this test.[2]

Indications

The indications of thyroid uptake and scan are as follows:

• Differential diagnosis of hyperthyroidism
• Suspected thyroid cancer
• Suspected metastasis of thyroid cancer
• Thyroid nodule
• Thyroid inflammation
• Determine the efficacy of radioactive iodine therapy
• Organification (incorporation of iodine into thyroglobulin) defects
• Determine congenital thyroid defects [3][4][5]

Potential Diagnosis

The thyroid uptake and scan have proved to help diagnose the following diseases:

• Graves disease.
• Toxic nodular goiter
• Toxic adenoma
• Thyroiditis
• Congenital defect of thyroid hormone synthesis
• Iodine deficiency
• The recovery from subacute, silent, or postpartum thyroiditis
• Destructive thyroiditis
  • Subacute thyroiditis
  • Silent thyroiditis
  • Postpartum thyroiditis [6]

Normal and Critical Findings

The normal thyroid uptake of radiotracer is 3 to 16% at 6 hours and 8 to 25% at 24 hours. These values may change according to laboratory standard techniques or patient dietary habits.

The thyroid gland can uptake more or less radioactive iodine than normal. More than normal uptake indicates a hyperactive thyroid, and less than normal uptake infers a hypoactive thyroid gland or interference with the uptake (see the list of potential conditions described below).

The following are some of the causes of increased uptake of radiotracer:

1. Hyperthyroidism due to Graves, multinodular goiter, or thyroid adenoma
2. Goiter
3. Early-stage of Hashimoto thyroiditis
4. Iodine deficiency
5. The recovery phase from subacute, silent, or postpartum thyroiditis
6. Pregnancy
7. Lithium carbonate therapy
8. Withdrawal of antithyroid medication
9. Rebound after the suppression of thyrotropin
10. Congenital defects of thyroid hormone synthesis

Some of the causes of decreased uptake of radiotracer are as follows:

1. Primary hypothyroidism
2. Central hypothyroidism
3. Destructive thyroiditis
   • Subacute thyroiditis
   • Silent thyroiditis
   • Postpartum thyroiditis
4. Excess iodine
5. Dietary supplements
6. Radiological contrast
7. Medications
   • Amiodarone
   • Antithyroid drugs
   • Perchlorate
   • Thiocyanate
   • Sulphonamides
   • Sulphonylurea
   • High-dose glucocorticosteroids
   • Topical iodine
8. Post-thyroidectomy
9. External neck radiation

Interfering Factors

Factors that clinicians should consider before conducting thyroid uptake and scan are as follows:

• Diarrhea that can decrease the absorption of the dye if given orally
• Head CT with oral or intravenous contrast within the past 2 weeks
• Unacceptable quantity of iodine in the diet
• Hypochloremia that can increase the absorption of the radiotracer
• Iodine-containing drugs (amiodarone)
• Thyroid hormone replacement and antithyroid drugs
• Chronic renal failure that impairs iodide clearance expands the iodide pool and lowers the %RAIU [7]

Complications

Radioactive iodine uptake and scan is a safe procedure. Some of the complications of this procedure are as follows:

• Pain at the injection site
• Hypersensitivity and anaphylaxis to radiotracer
• Exposure of the fetus or baby, if performed during pregnancy or lactating period [8]

Patient Safety and Education

The following precautions are necessary before performing a thyroid uptake scan:

• Pregnancy should be ruled out with either serum or urine pregnancy tests, and the clinician should ask the patient to avoid pregnancy for 6 months after radioactive iodine administration.
• The patient should avoid breastfeeding after performing this test as radioactive iodine can be secreted in breast milk.
• A trained professional should administer this test.
• The patient should receive counseling regarding physical contact safety measures, including avoiding the exposure of urine, stool, saliva, vomit, blood, and body fluids, as well as perspiration, for 48 hours.
• Patients should also avoid public transportation and sitting close to others.
• Patients should carry documents indicating the date, provider information, and radionuclide used while traveling through the port of entry within 4 months of the procedure.
• Patients should receive counseling on flushing twice after urinating or defecating for 24 to 48 hours after the procedure.

Clinical Significance

The thyroid uptake and scan play a central role in diagnosing thyroid diseases and abnormalities in thyroid function as it provides detailed information on the functions and anatomy of the thyroid gland. Different radioactive tracers function to detect or diagnose various thyroid diseases. Iodine, technetium pertechnetate, gallium-67, and fluoro-deoxy-glucose are all used. Relative uptake of iodine and technetium pertechnetate by focal thyroid nodule is labeled as warm, cold, or hot. Other uses of iodine include treatment of thyroid cancer, evaluation of residual/recurrent disease, evaluation of distant metastasis, and follow-up of patients after thyroidectomy.[9][10]