Loss-of-function mutations, nonsense mutations, and missense mutations are three of the most common. Nonsense and missense mutations cause the most severe phenotypes. Although FMO3 mutations account for most known cases of trimethylaminuria, some cases are caused by other factors. A fish-like body odor could result from an excess of certain proteins in the diet or from an increase in bacteria in the digestive system. A few cases of the disorder have been identified in adults with liver damage caused by hepatitis. In 2007 the evolution of the FMO3 gene was studied, including the evolution of some mutations associated with TMAU. Diagnosis
Measurement of urine for the ratio of trimethylamine to trimethylamine oxide is the standard screening test. A blood test is available to provide genetic analysis. The prominent enzyme responsible for TMA N-oxygenation is coded by the FMO3 gene. False positives can occur in the following conditions, where elevated TMA can be present in the urine without any underlying TMAU:

Urinary tract infection
Bacterial vaginosis
Cervical cancer
Advanced liver or kidney diseaseA similar foul-smelling odor of the urine has also been associated with colonization of the urinary tract with a bacterium called Aerococcus urinae, especially in children.Olfactory reference syndrome is a condition where there is a persistent false belief and preoccupation with the idea of emitting an abnormal body odor. According to McNiven at a canadian genetics clinic, 83% of referrals for genetic testing for TMAU were deemed likely to instead have ORS.
Trimethylaminuria (TMAU), also known as fish odor syndrome or fish malodor syndrome, is a rare metabolic disorder that causes a defect in the normal production of an enzyme named flavin-containing monooxygenase 3 (FMO3). When FMO3 is not working correctly or if not enough enzyme is produced, the body loses the ability to properly convert trimethylamine (TMA) from precursor compounds in food digestion into trimethylamine oxide (TMAO), through a process called N-oxidation. Trimethylamine then builds up and is released in the persons sweat, urine, and breath, giving off a strong fishy odor or strong body odor. Primary trimethylaminuria is caused by genetic mutations that affect the FMO3 function of the liver. A variant of TMAU (secondary trimethylaminuria or TMAU2) is an acquired form of TMAU where there is no genetic cause, yet excessive TMA is secreted. TMAU2 can be caused by a range of issues, including a precursor overload (ingesting too many precursors), hormonal issues related to menstrual cycles, liver damage, and liver and kidney failure. It is hypothesised that intestinal dysbiosis may cause temporary TMAU2 symptoms, however there is no direct evidence of this in a scientific study. Metabolic pathway
Trimethylamine enters the body via the consumption of certain foods and supplements. When food is consumed that contains TMA and/or TMAO (predominately seafood; saltwater fish, shellfish, seaweed and kelp), TMAO is converted by bacteria in the lower gastrointestinal tract (gut) into TMA.