Thyroid uptake of 201Thallium and its control by TSH

Abstract

Patients injected with 201Thallium (201Tl) for myocardial scanning present good thyroid visualization. Determinations in mice injected with 201Tl indicated a high thyroid/serum concentration ratio (T/S). The 201Tl biological half life (t\m=1/2\) in serum (30-135 s) was much shorter than in thyroid (53-55 h) for human subjects and experimental animals. The 1 h 201Tl T/S ratio was comparable to that of 131I and 99m Tc, indicating presence of a gradient for 201Tl also. Increase of endogenous TSH induced by propylthiouracil led to a significant rise in T/S for 99mTc, 131I and 201Tl, whereas TSH inhibition by feeding l-thyroxine led to decrease in T/S for 99mTc and 201Tl. In vitro thyroid/medium concentration ratio (T/M) of 99mTc and 201Tl was decreased after 20' incubation with ouabain, an inhibitor of the Na+,K+, activated ATP-ase. However, perchlorate in vitro or in vivo failed to diminish the 201Tl T/M ratios or to affect the t\m=1/2\of 201Tl in human subjects, whereas T/M of 201Tl was depressed by KCl addition to the medium. 201Thallium (201T1) is widely used for the visuali¬ zation of the normal and injured myocardium (Botvinick et al. 1978;Dunn et al. 1978). As an incidental finding it has been repeatedly observed that the thyroid was also visualized (Atkins et al. 1977;Palermo et al. 1979), suggesting a high deposition of 201T1 in the gland. This deposition has also been studied in experimental animals (Oster et al. 1978). The present study further explores the mechanism of action of this selective concentration of 201T1 in the thyroid gland of man and of experimental animals. Materials and MethodsMaterials. All radioisotopes were obtained commercially. 201T1C1 had a specific activity of no less than 1 mCi/ 180 ng, 131I was carrier free. 99mTc pertechnetate was generator-eluted. 42K had a specific activity of 1 mCi/ 13 mg. TSH-NIH-B7 was a generous gift from the NIH pituitary hormone distribution program. Methods I Human studies. Patients referred for myocardial scan¬ ning for diagnosis of cardiac pathology were injected iv with 1.5-2.0 mCi 201T1 and were informed that thyroid (and occasionally serum) radioactivity would be meas¬ ured, not as part of the cardiac study. In such patients, radioactivity over the thyroid region was counted with an uptake probe, at 75 kev ± 14%, 3-4 times for the first 6 h and subsequently up to a week at approximately 24 h intervals. Radioactivity-time distribution was plotted on semi-log paper, and thyroid and serum biological t'/2 were calculated. II Animal studies. Male or female mice of the CF1 strain weighed 20-35 g. They were fed Rat Chow and given tap water freely. Subsequently they were used for in vivo and in vitro procedures.A. In vivo procedures 1) 20lTl deposition in various organs. In a number of experiments, groups of mice were sacrificed at 30, 90, For reprints contact:

Nuclear Imaging in the Management of Thyroid Carcinoma

“…These potential causes of artifact include nasal secretions (23), salivary glands and saliva, gastric mucosa, large bowel, urine (on skin), and sites where sweat may pool (e.g., in braided hair) (3,24,25). The liver is often visualized on scans obtained following administration of large amounts of 131I, particularly 3 to 7 days following therapy (26,27). Radiolabeled thyroid hormone released into the blood from functioning thyroid tissue (normal or malignant) is concentrated in normal liver parenchyma.…”

“…The largest study was that of Hoefnagel and co-workers (36), who reviewed retrospectively experience with 201T1 and 131I scanning in 301 patients with differentiated thyroid carcinoma. Among a group of 226 patients with negative 131I scans (5 mCi), 26 had positive 201T1 scans, of which 23 were confirmed by biopsy. The authors reported a sensitivity of 131I scanning of only 48%, but sensitivity of the combination of 131I and 201T1 was 98%.…”