PAH and diabetes mellitus: Studies have shown that the presence of lower serum tyrosine levels in subjects with chronic renal function may be due to the insufficiency of phenylalanine and dysfunction of phenylalanine hydroxylase, resulting in impaired conversion of phenylalanine to tyrosine (52, 53), while the oxidative stress state accompanying the onset of diabetes limits the activity of tetrahydrobiopterin, further damaging phenylalanine hydroxylase activity (54).