Hormone-induced modifications of free tyrosine in the rat thyroid gland.

The concentrations of free tyrosine in liver, muscle, kidney and thyroid gland were determined in separate groups of rats maintained on a low iodine diet and treated with intraperitoneal injections of thyroxine (T 4), thyrotrophin (TSH) and TSH plus propylthiouracil (PTU). Groups of hypophysectomized rats also were given T 4. Significant changes in tissue tyrosine were generally confined to the thyroid gland. Animals treated with T 4 showed a decrease of mean thyroid tyrosine from 113.3 +/- 17.9 (S.D.) mug/g wet weight of gland to 76.2 +/- 5.7 mug/g (P < 0.01). Although there was little change in content when TSH was given alone, a significant increase in tyrosine levels was observed when TSH plus PTU were administered (P < 0.01). In hypophysectomized rats thyroid tyrosine decreased and was further lowered (to 46.0 +/- 3.1 mug/g; P < 0.05) by T 4 treatment. When tyrosine concentrations were expressed in relation to ribonucleic acid (RNA) or protein content of the assayed gland, all differences in tyrosine content became greater.