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Evaluation of the Resin Uptake of I¹³¹ Triiodothyronine as a Test of Thyroid Function¹

New York City

ABSTRACT

An in vitro test of thyroid function has been evaluated in a series of about 1000 measurements. The test consists in adding to the patient's serum a known amount of I¹³¹ labelled triiodothyronine, mixing this with an ion-exchange resin, and measuring the amount of radioactive iodine absorbed by the resin. All uptake or absorption values are related to that obtained with a sample from a large pool of mixed sera (chemistry laboratory or blood bank) which is assumed to represent the normal individual. On the basis of the reading of this sample as 1.0, values from 0.80 to 1.30 indicate euthyroidism. Above 1.30 the indication is for hyperthyroidism and below 0.80, for hypothyroidism.

In a series of 383 individuals clinically euthyroid, 3 per cent gave unduly high readings and 4 per cent unduly low. This is better than would be expected of the 24-hour I¹³¹ uptake test. Of 57 hyperthyroids, 14 per cent gave readings in the euthyroid range; of 72 clinically hypothyroids, 14 per cent gave readings in the euthyroid range; (this is comparable to the I¹³¹ uptake).

Adolescents and children give readings in the adult euthyroid region. The very young children tend to be in the lower part of the range; the teen-agers fall in the center.

Very early in pregnancy the value for the euthyroid woman drops into the low hypothyroid range and remains there until a few weeks post-partum.

It is not suggested that this test replace the 24-hour uptake, or any other currently employed test. It is an addition to the battery of tests available. It does have the outstanding advantage of not being influenced by previous medication with iodine. It is somewhat more difficult and time consuming than the standard uptake test, but no more so than the PBI.

FOOTNOTES

¹ From the Department of Radiology, College of Physicians and Surgeons, New York City.