3-Hydroxyisovaleric acid is a normal human metabolite excreted in the urine. It is a byproduct of the leucine degradation pathway. Production of 3-hydroxyisovaleric acid begins with the conversion of 3-methylcrotonyl-CoA into 3-methylglutaconyl-CoA in the mitochondria by the biotin-dependent enzyme methylcrotonyl-CoA carboxylase. Biotin deficiencies, certain lifestyle habits (smoking), or specific genetic conditions can reduce methylcrotonyl-CoA carboxylase activity. This reduction can lead to a buildup of 3-methylcrotonyl-CoA, which is converted into 3-hydroxyisovaleryl-CoA by the enzyme enoyl-CoA hydratase. Increased concentrations of 3-methylcrotonyl-CoA and 3-hydroxyisovaleryl-CoA can lead to a disruption of the esterified CoA:free CoA ratio, and ultimately to mitochondrial toxicity. Detoxification of these metabolic end products occur via the transfer of the 3-hydroxyisovaleryl moiety to carnitine forming 3-hydroxyisovaleric acid-carnitine or 3HIA-carnitine, which is then transferred across the inner mitochondrial membrane where 3-hydroxyisovaleric acid is released as the free acid. 3-Hydroxyisovaleric acid has been found to be elevated in smokers and in subjects undergoing long-term anticonvulsant therapy with carbamazepine and/or phenytoin. These levels are elevated due to impairment of renal reclamation of biotin. Levels may also be increased from prolonged consumption of raw egg-whites (OMIM: 210210, 253270, 600529, 253260, 246450, 210200, 238331). When present in sufficiently high levels, 3-hydroxyisovaleric acid can act as an acidogen and a metabotoxin. An acidogen is an acidic compound that induces acidosis, which has multiple adverse effects on many organ systems. A metabotoxin is an endogenously produced metabolite that causes adverse health effects at chronically high levels. Chronically high levels of 3-hydroxyisovaleric acid are associated with at least a dozen inborn errors of metabolism, including 3-hydroxy-3-methylglutaryl-CoA lyase deficiency, 3-methylglutaconic aciduria type I, biotinidase deficiency and isovaleric aciduria, dihydrolipoamide dehydrogenase deficiency, 3-methylcrotonyl-CoA carboxylase 1 deficiency, 3-hydroxy-3-methylglutaryl-CoA lyase deficiency, late-onset multiple carboxylase deficiency, holocarboxylase synthetase deficiency, and 3-methylcrotonyl-CoA carboxylase 2 deficiency. 3-Hydroxyisovaleric acid is an organic acid. Abnormally high levels of organic acids in the blood (organic acidemia), urine (organic aciduria), the brain, and other tissues lead to general metabolic acidosis. Acidosis typically occurs when arterial pH falls below 7.35. In infants with acidosis, the initial symptoms include poor feeding, vomiting, loss of appetite, weak muscle tone (hypotonia), and lack of energy (lethargy). These can progress to heart, liver, and kidney abnormalities, seizures, coma, and possibly death. These are also the characteristic symptoms of the untreated IEMs mentioned above. Many affected children with organic acidemias experience intellectual disability or delayed development. In adults, acidosis or acidemia is characterized by headaches, confusion, feeling tired, tremors, sleepiness, and seizures.