Determination of endogenous labelling of thyroid hormones during a radioiodide test in euthyroid goiter patients and hyperthyroid patients

The administration of a diagnostic dose of radioiodide, as in the radioiodide uptake test, results in the formation and secretion of radioactive thyroid hormones (endogenous labelling). In this report a method is described by which thyroid hormones are chromatographically isolated from serum, with the simultaneous assessment of the incorporated radioactivity over a period of time. A low-level counter was used to measure the small amounts of activity present. By this method the hormone phases during a radioiodide uptake test may be better evaluated. The present results show a good diagnostic discrimination between euthyroidism and borderline or definite hyperthyroidism (by a factor of 10 and 40, respectively). The influence of factors such as a small intrathyroidal iodide pool need, of course, to be considered. Hence, by taking into account such parameters as the intrathyroidal iodide pool, the hormone clearance etc, a technique for the measurement of thyroid hormone secretion can be developed. (orig.)