Application of the radioiodine uptake to the clinical evaluation of thyroid disease

Abstract

The radioactive iodine uptake (RAIU) merits a position of prominence in thyroid function testing. With reasonable care function testing. With reasonable care technical errors virtually can be eliminated. The routine use of precalibrated capsules (which should be counted prior to administration), and a routine precount of all patients are particularly emphasized. The availability of simple and reliable confirmatory TSH and T3 suppression tests augments the intrinsic value of the RAIU. A single RAIU value obtained any time between 15 and 30 hr after administration of the tracer usually provides all the information needed. The necessity for routine short-interval values has been overemphasized. “Normal” ranges for the RAIU should be deemphasized since almost any value is compatible with almost any state of thyroid function. The T3 suppression test is helpful for evaluating patients with suspected hyperthyroidism and marginally high values for RAIU and other tests of thyroid function. Euthyroid patients with Graves' disease in evolution, or nodular goiter, may also fail to suppress on T3. The TSH test is of value in the differential diagnosis of primary and secondary hypothyroidism. However, this test is useful more often in the evaluation of patients receiving thyroid hormone for dubious indications, or to detect impaired functional reserve capacity. RAIU values parallel thyroidal clearance of iodide. Although iodide clearance usually correlates with hormonal secretion there are exceptions, resulting either from drug ingestion or abnormal thyroid function. Correlation of RAIU values with the clinical picture is essential for proper interpretation.