Phenylalanine is an essential amino acid, meaning that the body is unable to form it sufficiently from other molecules and must be supplied in the diet. Phenylalanine is used in the synthesis of multiple important molecules, such as the thyroid hormone thyroxine, neurotransmitters dopamine and noradrenaline, and the skin pigment melanin. Since the catabolism of phenylalanine and tyrosine is conducted in the liver, their blood levels rise in liver failure. Elevated phenylalanine concentrations are also observed in renal failure and serious illnesses. It is a heat-sensitive molecule, and its concentration rises after 3 days of sample storage at room temperature.