Endocrine Abstracts

Introduction: Phenylketonuria (PKU) is a rare inherited metabolic disorder that leads to the toxic accumulation of phenylalanine (Phe) causing usually severe mental retardation and seizures. The basis of the lifelong therapy is low in natural-protein diet and Phe-free amino acid mixtures, that provide the daily necessary amino acids, vitamins and micronutrients, such as Iodine and Selenium. It is not well known, whether the adherence to the therapy can influence the thyroid function in early-treated adult PKU patients.

Method: A prospective, cross-sectional study was conducted to assess the thyroid function, ultrasound structure and Iodine status of early-treated adult PKU patients. Seventy-five PKU patients (age 18–41 years) were included in this study. Based on their blood Phe values they were divided into two groups, with the cut-off point being the upper limit of 600 μmol/l: on-diet (n=27) and loose-diet (n=48) group. Spot urine Iodine concentration (I) and Iodine/creatinine ratio (I/Cr), blood Phe, Tyr, free triiodothyronine (fT3), free thyroxine (fT4), fT3/fT4 ratio, thyroid-stimulating hormone (TSH), thyroglobulin, anti-thyroglobulin antibody, anti-thyroid peroxidase antibody were measured and thyroid ultrasound was performed. The results were compared between the two groups.

Results: None of the PKU patients have abnormal thyroid function. The prevalence of thyroid nodes seen on ultrasound was 21.3%. Blood Phe or Tyr even Phe/Tyr ratio were not associated with TSH, fT4, fT3 or fT3/fT4 ratio or with the antibodies. However, we observed a significant difference in the urine Iodine concentration and I/Cr ratio between the two groups (P<0.05, P<0.01 respectively). The loose-diet group based on the I concentration (median of 99.14 μg/l, 28.8% <50 μg/l, 51.1% <100 μg/l) could be classified as Iodine deficient subpopulation. Blood Phe correlated inverse with the I concentration and I/Cr ratio (r=−0.28, P<0.05; r=−0.37, P<0.01), while blood Tyr showed correlation with I/Cr (r=0.30, P<0.05). fT3, TSH, Thyroglobulin, thyroid antibodies and fT3/fT4 ratio showed no significant difference between the groups, however, fT4 was significantly higher in the on-diet group (P<0.05, on-diet: 14.15 pmol/l±1.9 S.D.; loose-diet: 13.23 pmol/l±1.3 S.D.).

Conclusion: The result of this study suggests that Iodine status is influenced by the adherence to therapy in early-treated adult PKU patients. However, thyroid function might not be affected in adult PKU.