Age affects the generation of serum thyronine-binding protein by rats fed a low protein-high carbohydrate diet

The feeding of a low protein-high carbohydrate (PM) diet to young (8-week-old) rats induces the production of serum thyronine-binding globulin. The effect of age on this process was studied in older (5-month-old) rats. In contrast to the findings in 8-week-old rats, no significant differences in the serum concentrations of T3 and free T3 and the percentage of free T3 (equilibrium dialysis) were observed between the 5-month-old rats fed the control diet and those fed the PM diet. There were also no significant differences between the two diet groups in the distribution of tracer [125I]T4 and [125I]T3 among the serum proteins as assessed by gel electrophoresis. The changes in thyronine-binding globulin and subsequent alterations in T3 binding previously observed in young rats fed a PM diet do not occur in older rats, suggesting that the age and perhaps the weight of rats must be considered in studies related to diet-induced effects on thyroid hormone economy.