Sir.—Phenylketonuria (PKU) is an inborn error of metabolism characterized by the inability to hydroxylate phenylalanine (PHE) to tyrosine (TYR). The condition is diagnosed by documentation of elevated serum PHE levels, and typically leads to moderate to severe mental retardation in its untreated form. Individuals with PKU who undergo early dietary PHE restriction usually achieve a full-scale IQ in the normal range.1
Serum PHE levels of 120 to 480 μmol/L are the typical target values in treatment and constitute good dietary control. Current clinical recommendations and research findings support the need to maintain dietary control throughout childhood.2-4 For various reasons, monitoring of serum TYR levels is not routine in many metabolic clinics. In our clinic, approximately 38% of patients with PKU who are younger than age 19 years have below-normal serum TYR concentrations (48 μmol/L). This is not clearly correlated with the kind of medical food or diet
MAZZOCCO MMM, YANNICELLI S, NORD AM, et al. Cognition and Tyrosine Supplementation Among School-Aged Children With Phenylketonuria. Am J Dis Child. 1992;146(11):1261–1264. doi:10.1001/archpedi.1992.02160230019009
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