Thyroid Scan - Uses, Preparation, Procedure, Instructions, and Interpretation

What Is a Thyroid Scan?

The thyroid scan (or thyroid nuclear scan) is a nuclear medicine imaging that uses radioactive materials to determine the structure and function of the thyroid gland. The radioactive materials are also called radiotracers or radiopharmaceuticals. The thyroid gland is an endocrine gland that is located in front of the trachea (windpipe). The thyroid gland contains two lobes connected by an isthmus. This gland produces two hormones such as thyroxine (T4) and triiodothyronine (T3) which maintain the metabolism of the body. The thyroid scan uses radioactive iodine to see the size, shape location, and function of the thyroid gland.

When Is a Thyroid Scan Recommended?

The thyroid scan is recommended for the following conditions:

• Diagnosis of thyroid cancer.

• Thyroiditis (swelling of the thyroid gland).

• Congenital thyroid disorders.

• Thyroid cancer metastasis (spread of cancer to other parts of the body).

• To determine the efficacy of the radioactive iodine therapy.

• Thyroid cysts and masses.

• Hyperthyroidism (a disease characterized by excessive production of the thyroid hormone).

• Hypothyroidism (a disease characterized by decreased production of the thyroid hormone).

• Goiter (abnormally increased growth of the thyroid gland).

• Determine the efficiency of the treatment, such as surgery, radiotherapy (radio waves used to kill cancer cells), and chemotherapy (anti-cancer drugs used to kill cancer cells).

What Are the Radioactive Tracers Used in Thyroid Scans?

The radioactive tracer is a protein or molecule that is attached to the radioactive material, which includes the following:

Commonly Used

• I-123 or Iodine-123.

• I-131 or Iodine-131.

• 99m- Technetium pertechnetate.

Rarely Used

• 111In-pentoxide.

• Thallium - 201.

• 99mTc- sestamibi.

I-123 is a more commonly used radiotracer because of its lesser half-life.

How a Person Gets Prepared for the Thyroid Scan?

• A person should inform the doctor about their past and present medical conditions and the regular medicines that they are taking during their initial consultation.

• The doctor advises avoiding taking foods containing iodine a week before the scan, which includes foods, such as:

  • Iodized salt.

  • Sushi.

  • Dairy products.

  • Shellfish.

• A person is also advised to stop taking the following medications:

  • Antihistamine (medicine to treat allergies).

  • Cough syrups.

  • Multivitamin supplements.

  • Supplements containing iodine.

• If the scan includes RAIU (radioactive iodine uptake), a person is asked to stop the following medications:

  • Estrogen.

  • Lithium.

  • Corticosteroids.

  • Barbiturates.

  • Phenothiazines.

  • Adrenocorticotropic hormone (ACTH).

• Thyroglobulin and thyroglobulin antibodies are assessed before the scan in thyroid cancer patients.

• A person must inform the doctor if they are pregnant or suspect of being pregnant, as babies in the womb are more sensitive to radioactive tracers.

How Is a Thyroid Scan Taken?

• For a thyroid scan or thyroid nuclear scan, the doctor may inject a small amount of radioactive tracer into the vein in the arm 30 minutes before the scan. Sometimes, a person is advised to take the radioactive tracer by mouth in the form of liquid or capsules 24 hours before the scan.

• During the scan, an individual is asked to wear the hospital gown and lie flat on the imaging table with their head tipped backward and their neck extended. This helps in imaging the thyroid gland efficiently. After that, the gamma camera will take a series of images from three different angles (anterior, left anterior oblique, and right anterior oblique) of the thyroid gland.

• The gamma camera detects the emission of gamma rays from the radiotracer and produces images of the thyroid gland on the computer screen. While taking the images, a person is asked not to move to obtain clear images.

• After the scan, an individual is asked to wait until the radiologist reviews the images for results. If the images are not clear, the scan will be repeated. This scan usually takes about 30 minutes or less. A thyroid scan often combines radioactive iodine uptake tests.

How Is the Radioactive Iodine Uptake (RAIU) Test Taken?

RAIU is performed at two different times. After injecting the radiotracer, one is asked to sit straight in a chair. The technician will place a probe over the thyroid gland, which determines the amount of thyroid hormone produced. This test is performed at four to six hours and at 24 hours after radiotracer administration. RAIU usually takes only a few minutes.

What Are the Instructions Given After the Thyroid Scan?

• Patients can continue their regular diet and activities after the scan.

• Patients should limit their contact with infants and pregnant women.

• Patients are advised to drink plenty of water which helps to flush out the tracers from the body.

• Patients should inform the doctor if they have pain or swelling at the injection site.

• They might be asked to flush twice after using the toilet for 24 to 48 hours after the scan to prevent others from unnecessary exposure to the radioactive material.

• The doctor advises the person to avoid pregnancy for six months after the scan.

• Breastfeeding women are instructed to avoid feeding for about 48 hours after the scan, as the radioactive materials may pass through the breast milk.

What Are the Factors That Affect the Efficiency of the Thyroid Scan?

• In patients having diarrhea, the radioactive materials might not be absorbed properly if taken orally.

• Inform the doctor if one has recently had a CT (computed tomography) scan using iodine-based contrast or other imaging using radioactive iodine.

• A person is asked to remove any objects or jewelry that interfere with the scan.

• A person is instructed not to eat a few hours before the scan as it may affect the result of RAIU.

• Very little or more quantity of iodine in the diet is recommended.

• Hypochloremia (low level of chlorine in the blood) causes increased absorption of radioactive materials.

• Chronic renal (kidney) failure results in impaired iodine clearance which lowers the percentage of radioactive iodine uptake.

• Iodine-containing medications, such as Amiodarone need to be avoided for at least one week before the scan.

• Patients under thyroid replacement medication or antithyroid drugs are advised to stop taking them for about five days before the scan.

What Are the Interpretations of Thyroid Scans?

• Normal Result: A normal thyroid scan does not show any abnormalities in the size, shape, location, and function of the thyroid gland. The normal value of thyroid uptake is 3 to 16 percent at six hours and 8 to 25 percent at 24 hours.

• Abnormal Result: Sometimes, the thyroid gland may show an increased or decreased uptake of the radiotracer.

Increased Uptake: Areas of increased uptake of radiotracer are called hot areas. It might be due to the following conditions:

• Hyperthyroidism (a condition in which the thyroid gland produces a high level of thyroid hormone).

• Goiter (thyroid gland enlargement).

• Iodine deficiency.

• Pregnancy.

• Withdrawal of antithyroid medication.

• Congenital disorders of the thyroid gland.

Decreased Uptake: Areas of decreased radiotracer uptake are called cold areas. It may be due to the following conditions:

• Primary hypothyroidism (a condition in which the thyroid gland produces a low level of thyroid hormone).

• Central hypothyroidism (a condition in which a disorder of the pituitary or hypothalamus causes decreased thyroid hormone levels).

• Excess iodine.

• Destructive thyroiditis (a condition in which antibodies are produced against the thyroid, destroying thyroid tissues).

• Iodine-based contrast.

• Post thyroidectomy (after the complete or partial removal of the thyroid gland).

• External neck radiation (radiotherapy of the neck).

• Medications such as Amiodarone, antithyroid drugs, Sulphonylurea, and topical iodine.

Conclusion:

The thyroid scan is a noninvasive diagnostic tool for thyroid gland abnormalities. It provides more unique details of the gland than other imaging techniques of the thyroid gland. As this scan requires only about 30 minutes or less, it usually does not require hospitalization. As the iodine-based radiotracers are only absorbed by thyroid tissues, a thyroid scan is safe and specific for diagnosing thyroid disorders.