The norepinephrine system is related to lethargy; dopamine and its relationship to motivation, and lack of dopamine and depression; serotonin and its relationship to grief, and that low serotonin can lead to extreme grief and shame, and higher serotonin levels can restore a sense of well-being. Physical and emotional pain are linked in intricate ways.
The health of the thyroid hormone system can affect susceptibility to or worsening of depression. It is also important to note the link between thyroid hormone and growth hormone.
Genetics and stress play a significant role in determining the likelihood of developing depression, and those who carry the HTTLPR gene need fewer bouts of intense stress to trigger a depressive episode.