If the VA already service-connected your chronic fatigue syndrome (also called CFS, ME/CFS, myalgic encephalomyelitis, chronic fatigue), it can cause or worsen other conditions that are ratable on their own. These "secondary" claims are where many veterans leave money on the table. Below is what a service-connected chronic fatigue syndrome commonly leads to, why, and what evidence each one takes.
A secondary claim only works if the primary condition (chronic fatigue syndrome) is already service-connected, and you have a nexus — a doctor's opinion that it is "at least as likely as not" the cause. Nexus letter sample and template →
The disabling, long-term nature of CFS commonly leads to secondary depression and anxiety, which VA can service-connect.
What you'd need: A mental-health diagnosis and a nexus opinion linking it to the CFS.
CFS and fibromyalgia overlap heavily; widespread pain alongside the fatigue may support a separate fibromyalgia diagnosis, especially for Gulf War-era veterans.
What you'd need: A rheumatology work-up documenting tender points/widespread pain and a diagnosis.