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

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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.
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