As PAH normally converts phenylalanine to tyrosine, the block occurs between phenylalanine and tyrosine. As phenylalanine cannot be converted to tyrosine it builds up in the blood, when it reaches high levels it is termed hyperphenylalaninemia. This is not the only result of this disrupted pathway as is shown below. As we can see, tyrosine is ordinarily used to produce thyroxine, dopa and subsequently melanin, in PKU the defective enzyme causes deficiency of all these products as well as tyrosine. The raised phenylalanine in the blood results in increased phenylpyruvic acid which is excreted in the urine in PKU, this is useful in diagnosis .
Thyroxine deficiency can lead to hypothyroidism, which can be treated with hormone replacement.