High phenylalanine, or hyperphenylalaninemia, refers to a serum concentration of phenylalanine greater than 120 umol/L, and requires dietary modification or medication depending on the specific cause. 1. Phenylalanine hydroxylase deficiency: Hyperphenylalaninemia induced by this cause can generally be reduced by adjusting the patient’s diet to lower the concentration of phenylalanine in the blood, such as maintaining a low-phenylalanine diet and regular monitoring of phenylalanine concentration. 2. Tetrahydrobiopterin deficiency: In addition to dietary adjustments, this cause of hyperphenylalaninemia also requires medication, such as levodopa and 5 hydroxytryptophan. Warm reminder, hyperphenylalaninemia exists a certain degree of harm, patients need to actively accept the treatment, to avoid affecting the health.