Radioactive iodine uptake (RAIU) study is a nuclear laboratory test performed to evaluate thyroid function. Its primary value is in its ability to assist in diagnosing hyperthyroidism, because the radionuclide is readily concentrated in the thyroid gland. The radionuclide 123I is administered orally, followed by scanning of the thyroid area. Clients with hyperthyroid conditions reveal an increased uptake of the radionuclide. The test is not as effective in the diagnosis of hypothyroidism. Uptake is expressed in percentages of radionuclide absorbed in a specific amount of time. This percentage is calculated by dividing the amount of uptake by the amount or dose administered. An increase of more than 35 percent in 24 hours indicates hyperthyroidism.⁷⁴ The test is usually performed in combination with thyroid hormone tests when other thyroid function studies do not provide a definitive diagnosis.

• Recent use of iodinated contrast media for radiographic studies, which would decrease the uptake of the radionuclide
• Ingestion of foods containing iodine or medications containing iodine, such as cough syrup, potassium iodide, vitamins, Lugol's solution, or thyroid replacement therapy, which would dec-rease the uptake of the radionuclide
• Medications such as propylthiouracil, corticosteroids, warfarin, antihistamines, sulfonamides, nitrates, tolbutamide (Orinase), or isoniazid, which would decrease the uptake of the radionuclide
• Severe diarrhea, which would affect absorption of the radionuclide dose
• Lithium, estrogen, TSH, barbiturates, and phenothiazines, which would increase the uptake of the radionuclide
• Iodine-deficient conditions, cirrhosis, or renal failure, which would increase the uptake of the radionuclide

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Indications

• Evaluating thyroid function:
  • A response of increased uptake of 20 percent absorption in 1 hour, 25 percent in 6 hours, and 45 percent in 24 hours indicates hyperthyroidism.⁷⁵
  • A response of decreased uptake of 0 to 10 percent over a 24-hour period indicates a hypothyroid state but not necessarily hypothyroidism because of the many interfering factors reducing the uptake.
• Monitoring response to therapy for thyroid dysfunctional states

Contraindications

• Pregnancy, unless the benefits of performing the study greatly outweigh the risks to the fetus
• Sensitivity to iodine in foods or medications

[Section Outline]

• Indications
• Contraindications

Nursing Care Before the Procedure

Client preparation is the same as for any nuclear laboratory test (see section under "Total Blood Volume Study") and nuclear scan study (see section under "Brain Scanning").

• Inform the client of the scanning schedule and explain that the study can take 24 hours.
• Instruct the client to restrict any iodine-containing foods and medications for at least 1 week before the study.
• Withhold all foods and fluids for 8 to 12 hours before the study, according to department policy.
• History should include previous thyroid studies, allergies to iodine, medication regimen, and results of thyroid hormone tests.

The client is given the radionuclide orally, unless a 2-hour scan is to be performed, which requires IV administration. The client is placed on the examining table in a supine position with the neck slightly hyperextended on a small pillow. The client is then requested to remain still during the procedure. The scanning is performed with a detector over the neck in 30 minutes to determine the gland's ability to take up the radionuclide, in 6 hours to determine the gland's ability to bind iodine, and in 24 hours to determine the gland's total uptake of the radionuclide. Subsequent imaging can be performed for information about the gland's ability to release the radionuclide.⁷⁶ Determination of the percentage of uptake is calculated using the amount of the dose and the amount absorbed in a specific time period.

Nursing Care After the Procedure

Care and assessment after the procedure are the same as for any nuclear scan study (see section under "Brain Scanning") and venipuncture for injection of the radionuclide (see Appendix I).

• Assess for fatigue, nervousness, increased appetite, increased sweating, or weight loss common in hyperthyroidism and caused by increased metabolic activity.
• Resume normal dietary intake and thyroid replacement therapy.
• Resume medication regimen if iodine-containing drugs have been withheld before the study.
• Provide a written schedule for the client's return to the department for scanning.