Studies on the Metabolism of Aromatic Amines in Relation to Altered Thyroid Function in Man1

Abstract

Studies of the metabolism of several aromatic amines were conducted in 9 thyrotoxic patients, 1 hypothyroid patient and 4 normal subjects rendered thyrotoxic by the administration of L-triiodothyronine. Elevated thyroid function was accompanied by decreased tissue levels of monoamine oxidase (MAO), as measured in biopsy specimens of jejunal mucosa; this was also indicated by elevated urinary excretions of tryptamine and tyramine. Withdrawal of thyroid hormone from the hypothyroid patient was associated with increased tissue MAO activity and a slight lowering of urinary tyramine. Measurements of plasma amino acids indicated a unique elevation of plasma tyrosine levels in thyrotoxic states; the reason for this has not been determined. Elevated urinary values for histamine were also found in some thyrotoxic patients. The urinary excretions of catecholamines and their metabolites were not increased in 6 thyrotoxic subjects, and in some cases the urinary levels of norepinephrine and of vanilmandelic acid appeared to be low. The administration of an inhibitor of amino acid decarboxylase to 2 thyrotoxic subjects and a MAO inhibiting drug to the hypothyroid patient was associated with only slight subjective improvement. It is suggested that altered amine levels could conceivably contribute to some of the manifestations of human thyroid dysfunction, particularly to those involving the central nervous system.