Influence of diet composition on radiothyroxine kinetics in brook trout, Salvelinus fontinalis (Mitchill)

Abstract

The peripheral metabolism of L-thyroxine (T₄) labelled with ¹²⁵I(*T₄) was compared in two groups of 2-year-old brook trout maintained on a 12-h photoperiod at 11–12 °C and fed once daily for 31 days at a level of 2.2% of body weight with either a high protein, high calorie (HP–HC) or a low protein, low calorie (LP–LC) diet.Ingestion of the LP–LC diet decreased the plasma T₄ level, reduced biliary–fecal excretion of *T₄, depressed the monodeiodination of *T₄ to labelled 3,5,3′-triiodo-L-thyronine (*T₃), and lowered the T₄ degradation rate. Trout fed this diet were also noted to have increased liver size and reduced terminal body weights relative to those fed the HP–HC diet.These data suggest that feeding a low protein, low calorie diet may depress thyroidal function, as well as the conversion of *T₄ to *T₃.