IQ of people with mental illness follows the same distribution as IQ of people without mental illness. People with severe mental conditions are known to have low IQ and people with mental conditions also have extremely high IQ.

In conclusion, there is no fix way to determine the IQ of people with mental illness.

Diagnostic symptoms of depression include: Depressed mood, Sleep disturbance, Loss of Interest (anhedonia), Guilt or feelings of worthlessness, Energy loss and fatigue, Concentration problems, Appetite/weight changes, psychomotor retardation or agitation and Suicidal ideation.

I think the worse symptom of depression is suicidal ideation because if eventually this patient carrys out this ideations ir may lead to death.