There are 59 health conditions that can contribute to depression and are often misdiagnosed for depression. Very often medical conditions develop slowly and it can take 10, 15 or 20 years until it progresses to a disease that can be diagnosed using standard lab tests. During this time when the disease is in a subclinical state people can have many unexplained symptoms and depression is one of them. 

Antidepressants often fail to help in these cases because the root cause of depression was never addressed. What helps is to get a proper diagnosis and treat the underlying medical condition.