Radiothyroxine metabolism in several freshwater teleost fishes

Abstract

The metabolism of intraperitoneally injected radiothyroxine labeled with 125I in 3′ and 5′ positions (*T4) was followed in several starved freshwater teleosts acclimated at 12–13 °C.Biological half-lives for initial loss of plasma PB125I (*T4) were 7.9 h (goldfish), 8.7 h (rainbow trout), 12.2–28.5 h (channel catfish), 19.2 h (sucker), 24.0–37.6 h (burbot), and 56.6–138.0 h (brown bullhead). Frequently, a single exponential decrease in plasma PB125I did not occur, and the usually large *T4 distribution spaces suggested that injected *T4 might not rapidly enter the circulation from the injection site. Plasma radioactive derivatives included 3,5,3′ triiodo-L-thyronine.Biliary *T4 excretion varied considerably between species and could relate to feeding habits.