Thyroid suppression test with L-thyroxine and [Tc-99 m] pertechnetate

OBJECTIVE The thyroid suppression test is still used in some centres as an adjunt in the diagnosis of autonomous functioning thyroid nodules. With the purpose of minimizing the disadvantages of the original T3 suppression tes t, we have evaluated the efficacy of a method using L-thyroxine as TSH supp ression agent and [Tc-99m] pertechnetate as radiopharmaceutical. DESIGN Open nonrandomized prospective study MATERIALS AND METHODS A control group of 15 normal volunteers (11 males, 4 females; 21-35 years, mean 26.4 years) and a patient group of 20 patients ( 18 females, 2 males; 27-83 years, mean 53.6 years) divided into 4 subgroups , were studied: 7 patients with autonomous functioning nontoxic nodules, 3 with autonomous functioning toxic nodules, 7 with Graves disease and 3 with nonautoimmune diffuse toxic goitre. Baseline thyroid uptake and imaging we re begun 20 minutes after an intravenous injection of 370 MBq (10 mCi) of [ Tc-99m] pertechnetate. This was followed by a single daily intake of 2 mu g /kg of L-thyroxine, for 10 days. Thyroid imaging and uptake were then repea ted. RESULTS In the control group [Tc-99m] pertechnetate uptake after L-thyroxin e suppression had a mean reduction of 75.8 +/- 7.69% (58-87%) in comparison to the baseline level. All subjects were euthyroid by clinical and laborat ory criteria and none complained of side-effects, despite significant suppr ession of TSH levels. In the patient group, thyroid uptake after suppressio n decreased in 10 patients (maximum reduction 39%), was unchanged in 2 pati ents and increased in the remaining 8 patients. CONCLUSION The method described was efficient for demonstration of autonomo us thyroid tissue, since none of the patients showed significant reduction of thyroid uptake after L-thyroxine suppression compared with the control g roup. This test was as effective as the original T-3 suppression test, but more convenient to the patient: no side-effects, ease of hormonal intake, l ow dosimetry and short stay in the nuclear medicine laboratory.