[pygy_]

https://en.wikipedia.org/wiki/GTP_cyclohydrolase_I

The deficiency causes problems with amino acid metabolism, which themselves mess up the brain.

The depression was a mere symptom of the underlying disease. When you read the DSM, you realize there are no mental illnesses, just syndromes: sets of signs and symptoms. Except in select few cases, the pathophysiology is unknown.

[podgaj]

That is what I tell everyone struggling with unknown health issues: There are no diseases, only symptoms.