I-123, I-131 and Tc99m: Differences, Similarities and Clinical Use

Thyroid radioactive imaging and therapy are foundational tools in clinical endocrinology, providing both diagnostic and therapeutic options for a variety of thyroid disorders. These techniques are primarily used to:

  • Assess thyrotoxicosis and determine its etiology

  • Identify autonomous thyroid tissue, including toxic nodules and functioning goiters

  • Guide and deliver treatment for certain causes of thyrotoxicosis, thyroid autonomy, and thyroid cancer

Understanding the distinct properties of the main radioactive agents is essential for choosing the appropriate study or therapy. The most commonly used agents are Iodine-123 (¹²³I), Iodine-131 (¹³¹I), and Technetium-99m (⁹⁹ᵐTc-pertechnetate). Each of these substances has unique radiation characteristics, half-lives, and cellular uptake mechanisms, making them suitable for different clinical scenarios.

In Summary:

  • Iodine-123 (¹²³I) is ideal for high-resolution diagnostic imaging due to its gamma emission and short half-life, which minimizes radiation exposure.

  • Iodine-131 (¹³¹I) emits both beta and gamma radiation, allowing it to be used therapeutically to ablate thyroid tissue in Graves’ disease, toxic nodules, and thyroid cancer, while still permitting limited post-therapy imaging. Its physical properties, however, result in lower-quality diagnostic images.

  • Technetium-99m (⁹⁹ᵐTc-pertechnetate) mimics iodine uptake via the sodium-iodide symporter (NIS), provides rapid, low-dose imaging, and is particularly useful in acute or outpatient settings, though it is purely diagnostic and not organified into thyroglobulin.

Choosing the right agent depends on the clinical question, balancing the need for diagnostic accuracy, radiation exposure, and therapeutic effect.

References

Ross DS, Burch HB, Cooper DS, Greenlee MC, Laurberg P, Maia AL, Rivkees SA, Samuels M, Sosa JA, Stan MN, Walter MA. 2016 American Thyroid Association Guidelines for Diagnosis and Management of Hyperthyroidism and Other Causes of Thyrotoxicosis. Thyroid. 2016 Oct;26(10):1343-1421. doi: 10.1089/thy.2016.0229. Erratum in: Thyroid. 2017 Nov;27(11):1462. doi: 10.1089/thy.2016.0229.correx. Erratum in: Thyroid. 2025 Sep;35(9):1097. doi: 10.1089/thy.2016.0229.correx2. PMID: 27521067.

Gharib H, Papini E, Garber JR, Duick DS, Harrell RM, Hegedüs L, Paschke R, Valcavi R, Vitti P; AACE/ACE/AME Task Force on Thyroid Nodules. AMERICAN ASSOCIATION OF CLINICAL ENDOCRINOLOGISTS, AMERICAN COLLEGE OF ENDOCRINOLOGY, AND ASSOCIAZIONE MEDICI ENDOCRINOLOGI MEDICAL GUIDELINES FOR CLINICAL PRACTICE FOR THE DIAGNOSIS AND MANAGEMENT OF THYROID NODULES--2016 UPDATE. Endocr Pract. 2016 May;22(5):622-39. doi: 10.4158/EP161208.GL. PMID: 27167915.

Sodium Iodide I 123. FDA Drug Label.

Sodium Iodide I 131 Diagnostic. FDA Drug Label.

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