Effects of Altered Thyroid Function in Rats Upon the Formation and Distribution of Tyrosine

The effects of altered thyroid status upon hepatic phenylalanine hydroxylase activity were studied in rats. After treatment with thyroxine, phenylalanine hydroxylase activity was unchanged; radiothyroidectomy produced a 27% decrease. These effects upon phenylalanine hydroxylase activity were minimal when compared to changes observed in hepatic tyrosine transaminase activity under similar conditions. Measui'ements of 14C-tyrosine disappearance from plasma demonstrated that treatment with thyroxine increased the fractional turnover rate of tyrosine. The “volume of distribution” of tyrosine in control rats was 1/3 greater than in thyroxine-treated rats. These studies suggest that the elevated concentration of tyrosine in plasma noted after administration of thyroxine is probably not derived from an enhanced conversion from phenylalanine, but more probably is related to the diminution in the “volume of distribution” of tyrosine, by mechanisms which have not been elucidated. The experiments provide further support for the hypothesis that thyroid hormone has selective effects upon aromatic amino acid metabolism. (Endocrinology77: 295, 1965)