2.1 - THYROID SUPPRESSION TEST WITH L-THYROXINE [TST4] AND [Tc-99m]PERTECHNETATE.

Ramos CD, Zantut-Wittmann DE, Tambascia MA, Assumpção LVM, Etchebehere ECSC, Camargo EE. Division of Nuclear Medicine, Department of Radiology and Department of Internal Medicine, Campinas State University (UNICAMP), Campinas, Brazil.

TST4 has been demonstrated as being as effective as the classical T3 and [I-131]iodide thyroid suppression test with the advantages of less side effects, ease of administration, low dosimetry and shorter duration. To evaluate the effectiveness of TST4 in demonstrating autonomous thyroid tissue, 20 patients (pts) with autonomous thyroid disease (18 females and 2 males; 27 to 83 years, mean 53.6 years) were studied. The pts were divided into 4 groups: 7 with non-toxic autonomous nodule (NTAN), 3 toxic autonomous nodule (TAN), 7 with euthyroid Graves´ disease but with laboratory proven active disease and 3 with diffuse toxic non-autoimune goiter (DTNAIG). Ultra-sensitive TSH, free-T4 and anti-thyroperoxidase and anti-thyroglobulin antibodies levels were obtained in all pts. Four pts were on metimazol. Twenty minutes after an intravenous injection of 370 MBq (10 mCi) of [Tc-99m] pertechnetate thyroid uptake and scintigraphy were performed. Thyroid uptake and images were repeated after 10 days of oral L-thyroxine (L-T4) (2 mg/kg, single dose). Baseline thyroid uptake ranged from 0.7 to 12% (mean 3.0%). After suppression thyroid uptake ranged from 0.5 to 10% (mean 2.8%), with an average reduction of 6.7%. Thyroid uptake decreased in 10 pts (maximum reduction = 39%, mean 21%), uptake was unchanged in 2 pts and increased in the remaining 8 pts.. In 5 of the NTAN pts (four with a solitary nodule and 1 multinodular) the extra-nodular parenchyma had reduced uptake, but was visible; however, after suppression the uptake of the extra-nodular parenchyma declined even further, while the nodular uptake remained unchanged. There was no significant difference in the images acquired after suppression in pts with TAN, Graves´ disease and DTNAIG. TST4 is an effective method to demonstrate autonomous thyroid tissue.