Radioiodine metabolism in the thyroid gland of the lizard Agama stellio

Abstract

Kinetics of radioiodine incorporation and disappearance under standard conditions of housing and maintenance were undertaken in Agama stellio, the commonest of the Israel lizards. Quantitative and qualitative measurements of radioactive iodine incorporation were made by: (1) periodic measurements of radioactivity over the thyroid area, using a collimated scintillation counter; (2) periodic scannings of the whole body using an isotopic scanner and (3) assay of tissues for radioactivity. Although there was activity almost immediately after 131I injection over the thyroid, it was not the site of the exclusive iodine accumulation. Most of the radioiodine was concentrated from the first moment after injection in the stomach, which apparently acted as a reservoir for the 131I. It was subsequently released into the circulation and taken up by the thyroid. Maximum thyroid uptake was found 5–7 days after 131I injection, followed by a plateau of 6–7 days and a gradual decline. Labeled MIT, DIT, T 4 and in some cases small amounts of T 3, were found in the glands by thin-layer chromatography. Labeled T 4 reached a maximum of 20% of the total radioiodine within 7 days.