The thyroid gland has a very important role in production of animal. In beef cattle, hyperthyroidism is beneficial to growth in rearing period, and hypothyroidism is beneficial to fat deposit in the meat in finishing period. Thyroidal state seemed to be reflected in concentration of serum component and amino acid in the blood. The present study was carried out to know the relation of thyroidal state to the serum concentration of blood component and free amino acid, using male rats of the Wistar strain. In the study, three experiments were carried out and preparation of L-thyroxine was injected subcutaneously in each experiment. Thyroxine decreases serum concentration of BUN, Chol, Ca and increases Tp, Gl and P. In the hypothyroidal condition, increased level of glycine in the serum was again proved in this study. In blood serum of animals injected with thyroxine is not always clear. The cause may be that the injection of thyroxine cause a decrease of endogenous secretion of thyroxine. Effect of thyroxine showed reflect of effects of exogenous thyroidal hormone and decreased secretion of endogenous thyroxine. Hypothyroidal condition caused an increase in concentration of methionine and tyrosine.