The initial presentation includes abnormal movements and developmental delay in the first few months of life. [invitae.com]

delay despite a phenylalanine-restricted diet. [pediatricmotordisorders.org]

delay, seizures, ataxia Treatment: Dietary phenylalanine and tyrosine restriction, ascorbate supplementation 4-Hydroxyphenylpyruvate dioxygenase Not genetic Biochemical profile: Elevated plasma phenylalanine and tyrosine Clinical features: Usually occurring [msdmanuals.com]

Increased Risk During Pregnancy According to an abstract from the American Academy of Pediatrics (AAP), elevated levels of phenylalanine -- even mildly elevated levels -- during pregnancy may increase the risk of developmental delays and growth delays [vitamins.lovetoknow.com]

Abstract Elevated maternal phenylalanine concentrations during pregnancy are teratogenic and may result in growth retardation, microcephaly, significant developmental delays, and birth defects in the offspring of women with poorly controlled phenylketonuria [pediatrics.aappublications.org]

Untreated individuals have very fair hair, eczema, a mousy odor of the urine and skin, and progressive mental retardation. Treatment consists of a diet low in phenylalanine. [medical-dictionary.thefreedictionary.com]

PMID: 11581453 Assay Assay and technical information Invitae is a College of American Pathologists (CAP)-accredited and Clinical Laboratory Improvement Amendments (CLIA)-certified clinical diagnostic laboratory performing full-gene sequencing and deletion [invitae.com]

Too much, as you mentioned, is associated with fear, anxiety, stress, and at extreme levels perhaps stroke, etc. [bluelight.org]