Radioactive Iodine Uptake Testing

Radioactive iodine uptake (RAIU) testing is a useful diagnostic tool for assessing thyroid pathologies. The RAIU test is a reliable measurement when using a dedicated probe with a reproducibility of 1% and a 95%-least-significant-change of 3%. ^[1]

The atom is the smallest unit of an element. An atom consists of electrons, protons, and neutrons. Electrons revolve around the nucleus (made of protons and neutrons) in fixed orbits. The number of electrons equals number of protons in a neutral atom. Electrons can be entirely removed from an atom by absorption of energies that exceed the binding energies of the electrons, causing the atom to be ionized. ^[2] These excited atoms can release energy in the form of electromagnetic radiation. Electromagnetic force manifests itself as waves or particles called photons. A nuclide that releases radiation is called a radionuclide.

In the nuclide (nucleus), the number of neutrons (N) plus the number of protons (Z) equal its mass number (A). In the case of iodine-131 (¹³¹ I), the atomic number Z (number of protons) is 53 and number of neutrons is 78. So the total mass number is 131. An unstable nuclide tries to attain stability through radioactive decay releasing alpha, beta and gamma rays. In medicine, this radiation is used for diagnosis and treatment.

A normal thyroid gland traps about 10–35% of ingested radioiodine through the sodium-iodide symporter (NIS). ^[3] NIS is also present in the salivary glands and breast, but only the thyroidal NIS is regulated by the thyroid-stimulating hormone (TSH). Thyroidal NIS cannot distinguish between radioactive and non-radioactive iodine (iodine-127). More than 20 radionuclide of iodine are recognized, but only iodine-123 (¹²³ I) and¹³¹ I are commonly used for clinical purposes. On certain occasions iodine-124 and iodine-125 are used.

Because¹²³ I has a comparatively short half-life and emits only (gamma) photons, it is ideal for diagnostic testing.¹³¹ I has a half-life of 8 days and emits beta rays as well as gamma rays. This makes it useful for diagnostic and therapeutic purposes. But the use of¹³¹ I for routine diagnosis is discouraged because the radiation dose is about 100 times stronger than that of¹²³ I.

Other noniodide radionuclides are used for the evaluation of patients with thyroid disorders include the following:

• Technetium pertechnetate (99mTcO4): This is more widely used than¹²³ I because of availability and cost. ^[4] Unlike¹²³ I, which is both concentrated and organified within the thyroid, technetium pertechnetate is only concentrated in the thyroid.

• 111In-pentreotide: This is a somatostatin analogue used in imaging medullary thyroid cancer and other neuroendocrine tumors. ^{[5, 6]}

• Thallium-201 (201T1)

• 99mTc-methoxyisobutylisonitrile (99mTc-sestamibi)

• 99mTc-tetrafosamin

See the images below.

Measuring thyroid uptake

Thyroid uptake is another measure of thyroid function. Measurement allows for the following:

• Diagnosis of hyperthyroidism

• Distinguishes other causes of thyrotoxicosis from hyperthyroidism

• Provides data for calculation of a therapeutic dose of¹³¹ I

• Detects intrathyroidal defects in organification

Thyroid imaging

See the list below:

• Most commonly used in the evaluation of hyperthyroidism

• Allows comparison of structure and function within the thyroid

• Can differentiate Graves disease from toxic adenoma, toxic multinodular goiter, or thyroiditis

• May aid in the differential diagnosis of a mediastinal or cervical mass because thyroid tissue may concentrate the diagnostic isotope

• Identifies ectopic thyroid, such as might occur from the foramen cecum down through the neck

• Assists in the diagnosis of congenital hypothyroidism

• Determines whether functioning metastases from thyroid cancer are present and amenable to¹³¹ I therapy

• Determines whether a patient with thyroid cancer has been successfully treated

Contraindications to radioactive iodine uptake (RAIU) testing include:

• Hypersensitivity reaction to iodine

• Pregnancy

• Breastfeeding

• Severe Grave ophthalmopathy

Special precautions are needed for handling and disposing the radioactive material from radioactive iodine uptake (RAIU) testing. ^[7]

Certain pathologies require certain concerns, as follows:

• Kidney disease: Kidney impairment can increase thyroid uptake of radioactive iodide.

• Hypochloremia: This can increase thyroid uptake of radioactive iodide.

• Cardiac disease related to thyroid disease: This can exacerbate radiation-induced thyroiditis.

Simultaneous use of iodine (including amiodarone), thyroid, or antithyroid drugs impedes the uptake of radio iodide uptake. Discontinue antithyroid therapy 3–4 days prior to¹³¹ I administration.

Using radioactive iodine treatment for hyperthyroidism in patients younger than 30 years is controversial, especially in women. Exclude pregnancy with either a urine or serum pregnancy test. Exclude radioactive iodine therapy during lactation. Diagnostic testing using tracer amounts can be performed, and the nuclear medicine service can actually test breast milk for the presence of the radio nuclide, allowing for safe resumption of breast feeding.

Best practices

Use antithyroid agents with or without beta-blockers before treatment with sodium iodide I¹³¹ to prevent symptomatic radiation thyroiditis. Treating the patient to a euthyroid state depletes the thyroid of thyroid hormone that would be otherwise released at the time of the treatment.

Whether to continue antithyroid drugs after radioactive iodine treatment is controversial. Some endocrinologists prefer not to do so, unless a patient is overtly toxic. The reason for this is a significant amount of recirculating radioactive iodine is released and then taken up again. Antithyroid drug therapy after the treatment minimizes this uptake of recirculating radioactive iodine and may minimize the outcome.

Hydrate patients adequately prior to dosing.

It may be used in children because the patient may not be compliant with meds or still need to be treated.

The iodine should only be handled and administered by trained professionals.

Practically speaking, most patients who get radioactive iodine are likely to develop hypothyroidism. Giving enough to prevent hyperthyroidism and still maintain thyroid function long-term is impossible. In fact, most people are given higher doses so as not to have another bout of hyperthyroidism complicate their medical course. In addition, patients need to be monitored carefully to prevent severe, symptomatic hypothyroidism after treatment.

Use of radioactive iodine therapy in women is controversial. This is because of exposure to the ovaries. In general, the ovaries receive the same radiation from treatment with radioactive iodine for Graves' disease, which they do for a barium enema or an intravenous pyelogram. This puts it in better perspective. In addition, the rate of teratogenicity after radioactive iodine in the mother is similar to the population at large. ^[8]

Patient education is very important about the common side effects and necessary steps taken to prevent or reduce the risk of exposure to others. This includes avoiding the following during the restrictive period: ^[9]

• Close contact with children and pregnant women

• Sexual contact

• Sleeping in the same bed with another person

• Sharing cups and utensils

The restrictive period depends on the dose administered and retained.

Procedure planning

These are recommendations set forth by the American Thyroid Association. ^[10] They are mainly related to the use of¹³¹ I because it is associated with the greatest radiation exposure. The recommendations are as follows:

• Radiation safety officer: The radiation safety officer must develop patient-specific precautionary measures, considering the predicted retained radioactivity in the patient. They must also make special rules and regulations in the administration of radiopharmaceuticals by a well-trained clinician.

• Pregnancy/reproduction: Women should be informed that pregnancy is a contraindication with¹³¹ I therapy. The guideline specifies women from age of menarche to 2 years after menopause should be tested for pregnancy. Women should be advised to avoid pregnancy for 6 months after treatment to allow for the normalization of thyroid levels. Men should be informed that full fertility may not return for 1 year. Fathering a child within 3 months of radiation exposure is also not recommended.

• Breastfeeding: This should be avoided.

• Dose rate calculations to determine distance: When the exposure from a patient who received treatment at 1 meter (3.2 feet) exceeds the Nuclear Regulatory Commission (NRC) defined regulatory limit of less than 7 millirem/h, the public should remain at least 6 feet (1.8 meter) away. Adults may come close up to 1 meter from the patient for few minutes.

• Travel through ports of entry (including airports, tunnels or bridges): Patients who have had treatment (within 4 mo) should carry documents indicating the date of treatment, radionuclide used, and provider contact information.

• Posttherapy accommodations: Patients should be advised to avoid public transportation, close proximity to other people, and staying in hotel.

• Personal hygiene: Safety measures should be undertaken during the first 48 hours to decrease the possibility of exposing others to urine, stool, saliva, blood or bodily fluids, perspiration, or vomit. Patients should use specialized leak-proof waste disposal bags during the restricted period.

Causes of increased iodine uptake include the following: ^[3]

• Hyperthyroidism

• Iodine deficiency

• Pregnancy

• Recovery phase of subacute, silent, or postpartum thyroiditis

• Rebound after suppression of thyrotropin

• Rebound after withdrawal of antithyroid medication

• Lithium carbonate therapy

• Hashimoto thyroiditis

• Congenital defects of thyroid hormogenesis apart from trapping defect

Causes of decreased uptake include the following: ^[3]

• Primary hypothyroidism

• Destructive thyroiditis (subacute thyroiditis, silent thyroiditis, postpartum thyroiditis)

• Post-thyroidectomy,¹³¹ I treatment, or external neck radiation

• Central hypothyroidism

• Thyroid hormone

• Excess iodine

• Dietary variations and supplements

• Radiological contrast

• Amiodarone

• Topical iodine

• Medications other than those containing iodine (eg, antithyroid drugs, perchlorate, thiocyanate, sulphonamides, sulphonylurea, high-dose glucocorticosteroids)