Understanding Trimethylaminuria (TMAU)

What is TMAU?

Trimethylaminuria (TMAU), also known as fish odor syndrome or stale fish syndrome, is a rare metabolic disorder characterized by the abnormal accumulation and excretion of trimethylamine (TMA), which results in a strong, offensive odor resembling that of rotting fish. The odor is present in urine, breath, sweat, and reproductive fluids, and no physical symptoms accompany the condition. Affected individuals appear normal and healthy, but the odor itself often leads to significant social and psychological difficulty. Symptoms are usually present from birth and may worsen during puberty.

Sources: StatPearls / National Institutes of Health; GeneReviews / NCBI Bookshelf

What causes TMAU?

The underlying cause of TMAU is usually a deficient or dysfunctional liver enzyme called flavin-containing monooxygenase 3, or FMO3. Normally, this enzyme converts trimethylamine, a compound produced by gut bacteria during digestion of foods like eggs, fish, and certain vegetables, into an odorless compound. When FMO3 doesn’t work properly, trimethylamine builds up in the body instead.

There are two forms of the condition:

Primary TMAU is inherited, caused by mutations in the FMO3 gene passed down from both parents (autosomal recessive inheritance).
Secondary TMAU can occur even without a genetic mutation. This happens when the body is overwhelmed by a large intake of substances like choline, which the enzyme normally processes; this is sometimes seen in people with conditions like Huntington’s or Alzheimer’s disease who take high doses of choline as part of their treatment.

Sources: NORD (National Organization for Rare Disorders); StatPearls / NIH

Symptoms

The primary and often only symptom of TMAU is a persistent body odor, typically described as fishy, that comes through sweat, breath, urine, and other bodily fluids. Symptoms of TMAU can worsen with fever, stress, physical activity, hyperventilation, and around menstruation.

Because there are no other physical symptoms, TMAU is often invisible to observers and can go unrecognized by physicians unfamiliar with the condition, sometimes leading to long diagnostic delays.

Source: Tidsskrift for Den norske legeforening (Journal of the Norwegian Medical Association)

How is TMAU diagnosed?

Diagnosis typically involves a urine test measuring the ratio of trimethylamine to its odorless byproduct, trimethylamine N-oxide, a measurement called FMO3 metabolic capacity. Genetic testing of the FMO3 gene can confirm a primary genetic diagnosis and distinguish it from secondary, non-genetic forms of the condition.

Sources: GeneReviews / NCBI Bookshelf; NORD

Current management approaches

There is currently no FDA-approved treatment for TMAU. Existing management strategies focus on reducing symptoms rather than correcting the underlying enzymatic deficiency:

Dietary changes: In people with mild cases, symptoms are often relieved by restricting foods containing choline and lecithin, such as eggs, liver, certain fish, beans, and soy products. Because choline is an essential nutrient, dietary changes are typically made under the guidance of a physician or dietitian rather than through self-directed restriction.
Gut-targeted antibiotics: Some people with more severe cases may be prescribed a gut-targeting antibiotic such as metronidazole, which reduces the intestinal bacteria that break down choline and trimethylamine precursors into trimethylamine.
Riboflavin supplementation: For individuals whose gene variants don’t completely eliminate FMO3 activity, riboflavin supplements may help maximize the enzyme’s remaining function.
Activated charcoal and copper chlorophyllin: These supplements can bind trimethylamine in the gut, reducing the amount available for absorption into the body.
Skin and hygiene care: Frequent washing with low-pH soaps, along with regular clothing changes, is often recommended alongside other treatments.

All of these approaches are used under medical supervision, and individual response varies significantly, some patients find meaningful relief, while others see limited benefit, which is part of why an approved, mechanism-targeted therapy remains an unmet need.

Source: NORD

Living with TMAU

The psychological and social impact of TMAU is often more burdensome than any physical symptom. Research published in the Journal of the Norwegian Medical Association notes that the psychosocial burden of trimethylaminuria can be considerable, with those affected experiencing an increased incidence of loneliness, social isolation, anxiety, and depression.

If you’re navigating a TMAU diagnosis, know that these experiences are well documented in medical literature, they are a recognized part of the condition, not a reflection of anything being wrong with you.

Source: Tidsskrift for Den norske legeforening