Abstract
Fasting serum Tyr levels are known to be low in hypothyroidism. The pathogenesis of hypotyrosinemia in hypothyroidism is complex and appears to involve a number of factors, such as reduced activity of hepatic phenylalanine-hydroxylase. In order to investigate this factor, serum Phe and Tyr levels were measured in 19 children with congenital hypothyroidism (CH) and 20 normal controls (C). The mean serum Phe levels in CH and C were 1.54 ± 0.36 and 1.83 ± 0.61 mg/dl, respectively. The mean serum Tyr levels in CH and C were 0.91 ± 0.33 and 1.49 ± 0.47 mg/dl, respectively. Although serum Tyr levels in CH were significantly lower than in C (P< 0.001), there were no significant difference in Phe levels in the two groups (0.05<P<0.1). The mean Phe/Tyr ratios in CH and C were 2.10 ± 0.84 and 1.47 ± 0.58 mg/dl, respectively. Although the ratio in CH was significantly higher than in C (0.001<P<0.01), in view of normal levels of Phe in CH, the observed difference in Phe/Tyr ratios does not confirm the existence of a deficiency in Phehydroxylase activity in hypothyroid children. In summary : (1) serum Tyr levels in CH are markedly lower than normal, (2) serum Phe levels in CH are within normal range, (3) serum Phe/Tyr ratios in CH are higher than normal, (4) decreased rate of conversion of Phe to Tyr is probably not the cause of hypotyrosinemia in CH.
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Zangeneh, F., Abolmolouky, L. & Bostan, T. Serum Phenylalanine (Phe) and Tyrosine (Tyr) levels in Congenital Hypothyroidism. Pediatr Res 15, 1556 (1981). https://doi.org/10.1203/00006450-198112000-00124
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DOI: https://doi.org/10.1203/00006450-198112000-00124
