Further Studies of Plasma Tyrosine in Patients with Altered Thyroid Function

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Abstract
Studies were undertaken to characterize the elevated levels of plasma tyrosine observed in patients with hyperthyroidism and in normal people ingesting thyroid hormones. When plasma free amino acid concentrations were determined in 8 thyrotoxic patients, only tyrosine and glutamic acid were found to be significantly altered. Elevation of plasma tyrosine levels could be rapidly produced (within 24 hr) by T3 administration to normal volunteers, and reverted to normal within 2 to 3 days after T3 was discontinued. When oral loads of tyrosine were given to 9 thyrotoxic, 13 euthyroid and 5 myxedematous subjects, the peak levels of tyrosine were the highest (109 μg/ml ±9.0 se) in the thyrotoxic group, significantly elevated above normal (39.0±4.8) in subjects given T3 (50.3±2.0), and lower than normal in myxedema patients (24.0±6.0). There was no difference in oral tolerance curves of tryptophan and, phenylalanine before and after T3 treatment of 7 normal subjects, nor did the oral tryptophan tolerance in 6 thyrotoxic patients differ from the curves obtained in normal volunteers. Possible use of measurements of plasma tyrosine in diagnosis of thyroid disorder is suggested.