Although in vitro measurements of serum thyroid hormones and particularly the sensitive TSH assay have in recent years taken over as first-line tests, measurement of the thyroid radioiodine uptake (RAI-U) still has a a place in the diagnosis of thyroid dysfunction in certain specific circumstances. The widespread availability of short-lived radionuclides (iodine-123 and Tc-99m) allows in vivo thyroid uptake and imaging to be carried out with much lower radiation exposure to the patient than in the past, when I-131 was the only radionuclide available. RAI-U testing is probably most useful in the differentiation of various types of thyrotoxicosis in those patients in whom clinical and laboratory findings do not clearly point to the etiology of the condition. Thus, the RAI-U will usually distinguish between true hyper-thyroidism and thyrotoxicosis caused by either a destructive, self-limited form of thyroiditis or ingestion of excess thyroid hormone (factitious thyrotoxicosis). Other clinical indications for the RAI-U include calculating the proper therapeutic dose of I-131 for hyperthyroidism, determining the presence of autonomous function of the thyroid in patients receiving thyroid hormone, and investigating the nature of intra-thyroidal defects in iodine metabolism. Excessive intake of iodine, most commonly in the form of medication or as radiographie contrast dyes, is the major cause of falsely low RAI-U results. This is one reason why the test is of no value in the diagnosis of atrophie thyroid failure. Another limitation of the RAI-U is the fact that elderly patients with hyperthyroidism, particularly with nodular goiter, commonly have RAI-U values within the normal range. Nevertheless, when the limitations are kept in mind, the RAI-U is a valuable adjunct to thyroid testing in selected cases.
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