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A Three Day, Double Isotope, l-Triiodothyronine Suppression Test of Thyroid Autonomy^1,2,

Philadelphia, Pennsylvania

ABSTRACT

A three-day test of thyroid autonomy has been described, utilizing l-triiodothyronine and two isotopes of iodine (¹³¹I and ¹²⁵I). The test separates patients with hyperthyroidism, due to autonomy from euthyroid patients with suppressible glands.

In 98% of euthyroid patients with suppressible glands, the 24-hour uptake fell by more than 30% from baseline values (16% or greater). In one of 12 patients with diffuse toxic goiter and only one of 26 patients with toxic nodular goiter was the fall in the 24-hour uptake greater than 30%.

Occult autonomy in diffuse or nodular goiters was detected by means of the post-T-3 scan. The overall fall in radioiodine uptake in these patients was no different from the suppressible control group. By means of the isotopic count ratio obtained through a focusing collimator, localized areas of autonomy were identified and studied.

An early uptake determination was found necessary to avoid misinterpretation of rapid iodine turnover (fall in 24-hour uptake), after T-3 administration, in some patients with overt autonomy.

In patients with hot nodules, whether euthyroid or hyperthyroid, the initial radioiodine uptake was usually normal (20/26 patients).

Autonomy was found to present a wide spectrum of functional appearances, ranging from an occult area in a diffuse or nodular goiter, discovered only by means of the suppression test, through the single hot nodule (with or without hyperthyroidism), to multiple nodules and generalized autonomy, always with the presence of suppressed tissue (except when occult, and in Graves' disease).

Occult autonomy was found in six of 25 selected patients with diffuse nontoxic goiter and in six of 14 selected functioning ("warm") nodules.

FOOTNOTES

¹ From the Department of Nuclear Medicine, Division of Radiology, Albert Einstein Medical Center, York & Tabor Roads, Philadelphia, Pennsylvania.

² Presented at the 13th Annual Meeting of the Society of Nuclear Medicine, Philadelphia, Pennsylvania, June 22–25, 1966.

³ Present address: Division of Nuclear Medicine, Temple University Hospital, Philadelphia, Pennsylvania 19140.