Assessing Thyroid Function with I-131 or I-123 Sodium Iodide Uptake

Iodine-131 (I-131) and Iodine-123 (I-123) are radioactive isotopes of iodine used in the field of nuclear medicine for diagnostic and therapeutic purposes, particularly for evaluating thyroid function. Both isotopes can be administered orally in the form of capsules or liquid.


• Iodine-131 (I-131) or Iodine-123 (I-123) sodium iodide in capsules or liquid form

Iodine-131 (I-131) and Iodine-123 (I-123) are radioactive isotopes of iodine used in the field of nuclear medicine for diagnostic and therapeutic purposes, particularly for evaluating thyroid function. Both isotopes can be administered orally in the form of capsules or liquid.

Iodine-131 (I-131) Sodium Iodide

I-131 is a beta and gamma-emitting radioactive isotope with a half-life of approximately eight days. It is primarily used for the diagnosis and treatment of hyperthyroidism and thyroid cancer. I-131 is absorbed by the thyroid gland in the same manner as non-radioactive iodine, which enables the evaluation of thyroid function and the visualization of the gland's structure.

For diagnostic purposes, I-131 is administered in low doses, typically ranging from 50-100 μCi (1.85-3.7 MBq) per patient. When used for therapeutic purposes, higher doses are given to destroy overactive or cancerous thyroid cells. However, I-131 has limitations due to its longer half-life and higher radiation exposure, which makes it less suitable for some diagnostic applications.

Iodine-123 (I-123) Sodium Iodide

I-123 is a gamma-emitting radioactive isotope with a half-life of approximately 13 hours, making it more suitable for diagnostic purposes than I-131 in some cases. The shorter half-life results in lower radiation exposure to the patient, and its gamma emissions enable better imaging quality.

I-123 is administered in doses ranging from 100-400 μCi (3.7-14.8 MBq) per patient for diagnostic purposes. It is absorbed by the thyroid gland similarly to I-131, allowing for the evaluation of thyroid function and structure. However, I-123 is not used for therapeutic purposes, as it does not emit beta particles, which are necessary for destroying thyroid cells.

Both I-131 and I-123 sodium iodide are valuable tools in the diagnosis and treatment of thyroid disorders. The choice between these isotopes depends on the specific clinical situation, balancing factors such as diagnostic accuracy, radiation exposure, and the desired therapeutic outcome.


• The thyroid uptake test measures the proportion of an orally administered radioactive tracer that accumulates in the thyroid gland at specific time intervals. Radioactive iodine thyroid uptake (RAIU) quantifies the thyroid's uptake.


Fasting for a minimum of 4-6 hours is required.

A withdrawal period is needed for thyroid hormones and antithyroid drugs.

Possible interactions with other medications should be considered.

Iodine contrast procedures should be limited or avoided.

A low-iodine diet may be recommended for 3-7 days before the test.

Special precautions should be taken for breastfeeding patients, children, and potential fetal exposure.

Dosage Administration

I-131 NaI: 50-100 μCi (1.85-3.7 MBq) per patient

I-123 NaI: 100-400 μCi (3.7-14.8 MBq) per patient

Dose calibration

Oral administration of capsules or liquid

Measurement Timings

Measurements can be started at 2, 5, 6, or 24 hours after administration.

Patient Positioning

Supine position with a slight neck extension

Uptake System

  • System calibration and quality control tests
  • Measure capsule count before patient administration
  • Capsules are swallowed with a small amount of water
  • Thyroid count measurements begin at 2 hours for I-131 and 6 hours for I-123, with additional measurements taken at 24 hours or other intervals determined by the department.

Data Processing

Special software programs are used for calculations

RAIU is calculated using the following formula:

RAIU = (Thyroid counts - background counts) / (Capsule counts - background counts) * 100%

Clinical Applications

• Determining the I-131 dosage for hyperthyroidism treatment due to Graves' disease or toxic nodular goiter.

• Differentiating between subacute or painless thyroiditis and factitious hyperthyroidism from Graves' disease and other forms of hyperthyroidism.

• Diagnosing and confirming amiodarone-induced hyperthyroidism.

• Suppression tests.

• Limited value in diagnosing hypothyroidism.

Additional Required Examinations

• Clinical examination before capsule administration to minimize radiation exposure.

• Blood tests: TSH, FT4, FT3, TPO antibodies, TRab, etc.


• The uptake test also supports the performance of the scan, and the administered dose allows for obtaining structural information.


• Reports should include patient, institution, and physician identification data, technical data related to radiopharmaceuticals, dosages, uptake system, and acquisition data.

• The estimated absorbed dose due to the examination should be noted.

• Reports should include the uptake value and the reference range of values, which can vary significantly among different regions, areas, and countries.

Normal Radioactive Iodine Uptake Values

• At 2-6 hours, the range is 3 to 8-15%.

• At 24 hours, the range is 5-35% in most regions of Northern America and 15-50% in many other parts of the world.

Causes of Increased RAIU

• Hyperthyroidism (Graves' disease, Plummer's disease-toxic adenoma, trophoblastic disease, pituitary resistance to thyroid hormone, TSH-producing pituitary adenoma)

• Nontoxic goiter (endemic, inherited biosynthetic defects, generalized resistance to thyroid hormone, some cases of Hashimoto's thyroiditis)

• Excessive hormonal loss (nephrosis, chronic diarrhea, hypolipidemic resins, diet high in soybean)

• Decreased renal clearance of iodine (renal insufficiency, severe heart failure)

• Recovery of suppressed thyroid (withdrawal of thyroid hormone and antithyroid drug administration, subacute thyroiditis, iodine-induced myxedema)

Iodine deficiency (endemic or sporadic dietary deficiency, excessive iodine loss as in pregnancy or in the dehalogenase defect)

• TSH administration

Causes of Decreased RAIU

• Hypothyroidism (primary or secondary); Hashimoto's thyroiditis

• Defect in iodide concentration (inherited "trapping" defect, the early phase of subacute thyroiditis, transient hyperthyroidism)

• Suppressed thyroid gland due to thyroid hormone (hormone replacement, thyrotoxicosis factitia, struma ovarii)

• Iodine excess (dietary, drugs, and other iodine contaminants)

• Miscellaneous drugs and chemicals

In conclusion, the assessment of thyroid function using I-131 or I-123 sodium iodide uptake is an essential diagnostic tool for understanding various thyroid conditions. This procedure involves administering radiopharmaceuticals, measuring the uptake, and evaluating the results to determine normal or abnormal thyroid function. It is crucial to follow the appropriate methodology, clinical applications, and reporting standards to ensure accurate results and an effective diagnosis.

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