Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) tetrahydrobiopterin deficiency ?
    ### Response:
    Tetrahydrobiopterin deficiency is a rare disorder characterized by a shortage (deficiency) of a molecule called tetrahydrobiopterin or BH4. This condition alters the levels of several substances in the body, including phenylalanine. Phenylalanine is a building block of proteins (an amino acid) that is obtained through the diet.
    