Thyroid Uptake Studies with I¹³¹ in Very Small Doses

The 24-hour iodine-uptake determination is the most commonly used radioisotope study. At the present time doses up to 100 µc of I¹³¹ are given to the patient for this measurement. For an individual having a typical uptake of 20 per cent, the radiation to the thyroid gland in rems is about equal to the number of microcuries of the administered dose. Although this amount of radiation is not considered dangerous, it is always desirable to reduce the dosage to normal tissues whenever possible. This report will discuss the results of a preliminary study using only 0.1 to 0.2 µc of I¹³¹. The resultant radiation to the thyroid gland is then comparable to the yearly background radiation. So small a dose would thus make it reasonable to perform I¹³¹-uptake measurements routinely on normal persons or children. In patients for whom two consecutive uptake determinations are desired, as when TSH stimulation or T3 suppression is used, the initial measurement could be made with the small dose and the second measurement with the standard dose. The accuracy of this second measurement is thus increased, since the residual I¹³¹ from the small dose can be ignored.

I¹³¹ uptake is usually measured in this laboratory by administration of a tracer dose of about 8 µc of I¹³¹. Twenty-four hours later a scintillation crystal is placed 25 cm. from the neck and the radioactivity at this point is counted for one minute. In order to eliminate those counts which did not come from the thyroid, the patient is recounted for one minute with a 1/2-inch-thick lead shield (B shield) over the thyroid area. The difference of the two measurements is called the “net patient count.” A standard dose, identical with that given to the patient, is counted in a neck phantom under the same conditions. A one-minute background count is also taken of the phantom, with the standard dose removed. The difference of these two counts is the “net standard count.” The ratio of the net patient count to the net standard count times one hundred equals the per cent iodine uptake as in Formula 1.

Routinely a scintillation crystal 3 inches in diameter by 1 1/2 inches long, of NaI(T1) is used, and the output of the detector goes to a pulse-height analyzer which is adjusted to count those gamma rays originating from the main 0.364 Mev peak in I¹³¹. Because of clinical demands on the equipment, the small-dose uptakes in this study were performed on another unit using a detector 2 inches in diameter by 2 inches long NaI(T1).

If a patient is recounted immediately after the first measurement, a different result will usually be obtained. This variation is due to the statistical nature of the radioactive disintegrations. If all other factors are held constant, the variation due to this cause can be evaluated directly by Formula 2.

In essence, the standard deviation indicates that there is a 2 out of 3 chance that the answer obtained is within one standard deviation of the “true” answer.