Isotope scanning demonstrates thyroid activity distribution and is principally useful in patients with thyroid toxicity and nodularity.
Technique and limitations
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A low dose of radiolabelled iodine-123 or the cheaper technetium-99m demonstrates the distribution of activity throughout the thyroid gland.
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Routine isotope scanning is unnecessary and inappropriate for distinguishing benign from malignant lesions: 80% of cold swellings are benign, while 5% of functioning or warm swellings are malignant.
Toxic nodular disease
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The principal value of isotope scanning is in a toxic patient with a thyroid nodule or nodularity.
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A toxic nodule shows localised overactivity with suppression of the remainder of the gland.
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Toxic multinodular goitre shows several areas of increased uptake. Distinguishing these patterns has implications for therapy.
Demonstrating metastases
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Whole-body scanning is used to demonstrate metastases.
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All normally functioning thyroid tissue must first be ablated by surgery or radioiodine because metastatic thyroid cancer tissue cannot compete with normal tissue for iodine uptake.