Hormone

Serotonin

https://longcovidatlas.org/glossary/serotonin

In plain language

What it is

A signalling molecule, most of it made in the gut, that helps carry messages along the nerve route from gut to brain.

Why it matters

Long COVID can lower serotonin, and that drop is one of the better-supported explanations for brain fog and cognitive symptoms.

Think of it like

A messenger that starts in the gut. Send fewer messengers and the message that reaches the brain arrives faint.

For specialists

Formal definition

5-hydroxytryptamine, a monoamine neurotransmitter; roughly ninety percent is synthesised by enterochromaffin cells in the gut.

Mechanism

Reduced circulating serotonin, proposed to follow gut and immune changes and impaired tryptophan absorption, weakens vagal signalling to the brain in a way that tracks with cognitive symptoms.

Sources

Serotonin metabolites and the gut-brain axis in post-viral illness free full text ›

Related terms

Gut dysbiosis Dysautonomia