A three day, double isotope, l-triiodothyronine suppression test of thyroid autonomy.

In 1965 Werner and Spooner described a test of thyroid autonomy which has found wide acceptance in medical practice (1). They found that administra tion of l-triiodothyronine (T-3) in doses of 75 to 150 mcg daily, for seven days, suppressed the thyroid uptake of 131!in normal persons and in most patients with nontoxic goiter. Little or no change occurred in patients with diffuse toxic goiter (Graves' disease) and toxic nodular goiter. These findings have been amply con firmed by others (2-17). Thyroid extract and 1-thyroxine have a similar action. In these tests: (1) T-3 is administered for two to seven days prior to the second uptake; (2) The uptake is usually determined at 24 hours only; (3) Thyroid scanning is not usually done in conjunction with the uptakes, and when done, only one isotope (131!) is employed for both preand post-T-3 scans. A number of deficiencies in the original method became apparent when the technique was applied to evaluation of localized areas of thyroid autonomy: (1) Hormone administration for seven days prior to the second uptake makes testing of hospitalized patients too time consuming to be practical; (2) The use of only one isotope in goitrous patients with nonuniform iodine deposition produees considerable difficulty in accurate interpretation of localized areas of poor suppression on the second scan because of residual radioactivity from the initial dose; (3) The use of 100 microcuries of 131! for the initial scan necessitates an equal or greater dose for the second scan, which results in an unduly high radia tion dose to the thyroid gland.