Trimethylaminuria

Trimethylaminuria is a metabolic disorder in which trimethylamine accumulates and is excreted in sweat, urine, and breath, producing a fish-like odor. It most often results from impaired activity of the liver enzyme FMO3.

Trimethylamine: Trimethylamine is a volatile nitrogen-containing compound with a strong fish-like odor. Its buildup causes the characteristic odor in trimethylaminuria.
FMO3 gene: The FMO3 gene encodes flavin-containing monooxygenase 3, an enzyme involved in processing nitrogen- and sulfur-containing compounds. Pathogenic variants in this gene commonly underlie inherited trimethylaminuria.
Trimethylamine-to-trimethylamine N-oxide ratio: The urinary trimethylamine-to-trimethylamine N-oxide ratio compares the two compounds to assess trimethylamine oxidation. A high ratio can support diagnosis of impaired trimethylamine oxidation.
Olfactory reference syndrome: Olfactory reference syndrome is a psychiatric condition involving persistent distressing belief that one emits an offensive odor, often without objective evidence. It can resemble trimethylaminuria in reported odor concerns but has a different cause.
FMO3: FMO3 is a liver enzyme that oxidizes trimethylamine into odorless trimethylamine N-oxide. Reduced FMO3 activity is the main cause of inherited trimethylaminuria.
Autosomal recessive inheritance: Autosomal recessive inheritance occurs when a condition usually requires disease-causing variants in both copies of a gene. The inherited form commonly appears when both FMO3 gene copies are impaired.
Urinary trimethylamine test: A urinary trimethylamine test measures trimethylamine and its oxidized product in a urine sample. It helps distinguish persistent metabolic impairment from odor without elevated trimethylamine.
Bromhidrosis: Bromhidrosis is unpleasant body odor caused by microbial breakdown of sweat or other skin secretions. It is a nonmetabolic cause of odor that can be confused with trimethylaminuria.
Trimethylamine N-oxide: Trimethylamine N-oxide is a water-soluble metabolite formed when trimethylamine is oxidized. This normally odorless product is the main route for clearing dietary trimethylamine.
FMO3 deficiency: FMO3 deficiency is reduced activity of the enzyme that converts trimethylamine into trimethylamine N-oxide. It provides the biochemical link between inherited variants and trimethylamine accumulation.

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Body odorRelated: It illustrates how a specific metabolic defect can cause distinctive body odor.