Thyroid uptake of 201thallium and its control by TSH

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 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 99mTc, indicating presence of a gradient for 201Tl also. Increase of endogenous TSH induced by propylthiouracil led to a significant rise in 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 1/2 of 201Tl in human subjects, whereas T/M of 201Tl was depressed by KCl addition to the medium.