Bone Mineral Density in Adults With Hyperphenylalaninemia on Kuvan Therapy

Inclusion Criteria: * Diagnosis of hyperphenylalaninemia at birth * Age between 18 and 50 years * Participated in study HSC-MS-110-0262 Exclusion Criteria: * Peri-menopausal and menopausal women will be excluded because this is a time of increased bone loss related to hormonal mediated factors. * Patients taking bisphosphonates because it alters bone density. Therefore, bone mineral density would reflect the biphosphonate intervention rather than their true status. * Pregnant women due to the hazard of radiation exposure during a DXA scan. In addition women who have been pregnant or who have breastfed within one year of study enrollment will be excluded because these are periods of rapid bone loss which would not reflect the entity under study but would serve to confound the data.