In normal conditions, tyrosine is a non-essential amino acid, since it is formed sufficiently by the body from phenylalanine. However, when phenylalanine intake is insufficient, or phenylalanine conversion to tyrosine is impaired in conditions such as chronic kidney disease, the blood tyrosine concentration falls. Since the catabolism of phenylalanine and tyrosine is conducted in the liver, their blood levels rise in liver failure. Tyrosine is a heat-sensitive molecule, and its concentration rises after 4 days of sample storage at room temperature.