Slow fractional removal of nonextractable iodine from rat tissue after injection of labeled l-thyroxine and 3,5,3′-triiodo-l-thyronine: A possible clue to the mechanism of initiation and persistence of hormonal action

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Abstract

Previous studies have shown that a small but significant proportion of radioiodine from labeled L-thyroxine (T4) and 3,5,3′-triiodo-L-thyronine (T3) is incorporated into plasma and tissue proteins and is not, therefore, extractable with ethanol or other organic solvents. Other studies have shown that the complex consists, at least in part, of the iodothyronine in apparent covalent linkage with protein. In the present series of experiments the disappearance rate of nonextractable iodine (NEI) was determined in plasma, liver, and kidney after the injection of rats with a single dose of T4 and T3 labeled with radioiodine in the phenolic ring. The t½ of NEI decay was substantially longer than the t½ of the initial metabolic removal of T4 (16 hr) and T3 (4-6 hr). Thus, between days 3 and 11 the average t½ of plasma NEI derived from T4 was 2.2 days, from T3, 1.9 days; kidney NEI from T4, 7.4 days, from T3, 6.1 days; hepatic NEI from T4, 4.3 days, from T3, 5.2 days.

Authors

Jack H. Oppenheimer, Martin I. Surks, Harold L. Schwartz