Metabolic rate and thyroid status in rats fed diets of different protein-energy value: the importance of free T3

Oxygen consumption and thyroid hormone status have been investigated in relation to dietary protein and energy intake in young growing rats fed a control diet of 18% protein as well as diets containing 9 and 4.5% protein fed ad libitum and an 18% protein diet fed at a restricted intake for up to 14 days. Measurements of growth rate, food intake and oxygen consumption indicated that the rats fed a 4.5% protein diet ad libitum and those fed restricted amounts of an 18% protein diet have behaved similarly with severely arrested growth and reduced rates of oxygen consumption and food intake. However, the rats fed the 9% protein diet ad libitum differed from the control group only in terms of a reduced growth rate. Among all the rats thyroid hormone status was associated with the metabolic rate only in the case of the concentration of free L-3,3',5-triiodothyronine (T3), which was reduced in the groups fed 4.5% protein and 18% protein (restricted). Total T3 was actually increased in the rats fed 9% and 4.5% protein and unchanged in the restricted group. These differences in the concentrations of free and total T3 appeared to reflect the fact that the binding capacity for T3 increased with decreasing levels of dietary protein. It is concluded therefore that measurement of free rather than total T3 is the appropriate index of physiological thyroid status in protein-energy malnutrition, at least in the rat.