Growth and plasma thyroid hormone concentrations of chicks fed diets deficient in essential amino acids

Consumption of low protein (10%) diets is known to produce elevations in plasma triiodothyronine (T3) in growing chickens. Therefore, we evaluated the effect of individual essential amino acid deficiencies on plasma thyroid hormone concentrations. For 13 to 15 d, chicks were fed either a control diet free-choice, one of six amino acid-deficient diets free-choice, or the control diet, pair-fed at the level consumed by chicks fed each of the deficient diets. The control diet was a 50/50 mixture of broiler starter and purified amino acid diets. The amino acids, fed at the indicated percentages of National Research Council recommendations, were: arginine, 60%; lysine, 60%; threonine, 60%; leucine, 75%; isoleucine, 75%; and methionine, 50%. Feed consumption and weight gain were significantly lower in all deficient groups than in the free-choice control group. In all cases except leucine, deficient chicks also gained less weight than their pair-fed controls. Plasma T3 levels in the groups deficient in arginine, lysine, isoleucine, or methionine were higher than in their respective pair-fed controls. However, only with the isoleucine deficiency did T3 levels exceed those of control chicks given free access to feed. Thyroxine levels were significantly lower than control levels only with the lysine deficiency. These results suggest that changes in circulating levels of thyroid hormones in a protein deficiency may be a consequence of selected amino acid deficits, because individual essential amino acids, when deficient in the diet, do not exert the same effect on circulating levels of thyroid hormones.