Metabolic Rate and Thyroid Status in Rats Fed Diets of Different Protein-Energy Value: The Importance of Free T3

Abstract

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.

The Endocrine Response to Dietary Protein: the Anabolic Drive on Growth

“…In addition. as we have described [9]. concentrations of free T 3 in the protein-deficient rat change in the opposite direction to total T 3 due to increases in binding proteins.…”

Effect of Starvation and Subsequent Refeeding on Thyroid Function and Release of Hypothalamic Thyrotropin-Releasing Hormone

“…Fasting-induced changes in FT4 levels were most significant in female rats, and the decrease in FT4 levels in male rats was only appar ent after prolonged fasting (table 1). Serum total T3, the free T3 fraction, and serum free T3 all decrease during fast ing [1,42], Also, in rats fed a low protein, high carbohy drate diet, the serum free T3 fraction decreases [43][44][45][46], These changes are due to an increased production o f a T3-binding globulin [41][42][43] and are most significant in young rats [45].…”

Circulating Thyroid Hormone Levels in Young Pregnant Rats and Their Fetuses: Effect of Malnutrition

“…As shown in figure 1, total serum proteins of young malnourished preg nant rats were lower than in the virgin con trols; this might imply higher amounts of free circulating thyroid hormones (especially T3) in these animals. On the other hand, Cox et al [21] found low basal metabolic rates associated with high total (but low free) se rum T3 levels in young rats fed a low (4%)-protein diet during 14 days, suggesting that in young rats maintained on a low-protein intake there is an increase in the serum bind ing capacity of T> There are conflicting data in the literature with regard to the effects of malnutrition on the thyroid function of adult rats [22,23].…”