Effects of dietary phenylalanine and tyrosine on circulating thyroid hormone levels and growth in the chick

Studies were conducted to investigate the effects of a phenylalanine-tyrosine deficiency on serum thyroid hormone concentrations and growth in chicks from 0 to 14 days of age. Birds fed diets severely deficient in aromatic amino acids (AAA), lysine or valine were found in most trials to have reduced serum concentrations of either 3,5,3'-triiodothyronine (T₃) or 3,5,3',5'-tetraiodothyronine (T₄). Approximately one-half of the samples assayed did not show detectable levels of 3,3',5'-triiodothyronine (reverse T₃). The reduction in serum thyroid hormone levels was not observed until chick growth was markedly affected and was not specific for AAA deficiency. Phenylalanine alone could not efficiently supply tyrosine when dietary AAA were suboptimal as evidenced by significantly greater weight gains in chicks fed 50:50 or 55:45 (mole:mole) mixtures of phenylalanine and tyrosine as compared with birds fed the same total amount (moles) of AAA supplied solely by phenylalanine. However, at or above requirement levels, phenylalanine could completely meet the chick's demand for tyrosine. Serum tyrosine concentrations generally reflected the dietary levels of AAA.