I don't know whether you are talking about PTSD or depression. It is good to ask the question: "And what causes hypoglycemia". There are many causes but one is the over-consumption of refined carbohydrates, such as Sugar and sugary drinks. Excess glucose consumption may be responsible for the generation of free radicals, which can attack out DNA and cause damage to our body.
The body can protect itself against excess glucose consumption by creating insulin resistance. This means the receptors for insulin stops the transfer of glucose (and other nutrients) into mitochondria of the cells. There may also be a family predisposition towards diabetes or insulin resistance.

This may lead to Syndrome X or the Metabolic Syndrome, a cluster of symptoms due to high triglycerides, high insulin, obesity and may ultimately lead to diabetes type II. Hypoglycemia is the forerunner of diabetes. It means the body cannot convert energy sources (such as glucose) into biological energy. Without that energy the body cannot convert food substances (such as tryptophan) into serotonin, and people become depressed and obese.

Thus hypoglycemia is a major factor in the development of mood disorders.

Does this explain it?