The thyroid gland in fish mainly secretes the thyroid prohormone 3,5,3 0 ,5 0 -tetraiodothyronine (T 4 ), and extrathyroidal outer ring deiodination (ORD) of the prohormone to 3,5,3 0 -triiodothyronine (T 3 ) is pivotal in thyroid hormone economy. Despite its importance in thyroid hormone metabolism, factors that regulate ORD are still largely unresolved in fish. In addition, the osmoregulatory role of T 3 is still a controversial issue in teleosts. In this study, we investigated the regulation

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... of the ORD pathway by cortisol and T 3 in different organs (liver, kidney, and gills) of Solea senegalensis and the involvement of T 3 in the control of branchial and renal Na C , K C -ATPase activity, a prime determinant of the hydromineral balance in teleosts. Animals were treated with i.p. slow-release coconut oil implants containing cortisol or T 3 . Hepatic and renal ORD activities were up-regulated in cortisol-injected animals. T 3 -treated fish showed a prominent decrease in plasma-free T 4 levels, whereas ORD activities did not change significantly. Branchial and renal Na C , K C -ATPase activities were virtually unaffected by T 3 , but were transiently up-regulated by cortisol. We conclude that cortisol regulates local T 3 bioavailability in S. senegalensis via ORD in an organ-specific manner. Unlike T 3 , cortisol appears to be directly implicated in the up-regulation of branchial and renal Na C , K C -ATPase activities.