Alterations in neurotransmitters such as norepinephrine, serotonin, and dopamine contribute to depressive symptoms.

Increased or decreased levels of thyroid hormone can also alter mood in subtle or dramatic ways.

Antidepressant medications appear to relieve symptoms by reversing abnormal levels of neurotransmitters and hormones.

But that’s not the whole story.

Investigators have reported that depressed patients may also have elevated levels of cytokines — immune-system compounds that accelerate inflammatory reactions. In a recent study published by JAMA Psychiatry, Dr. Jeffrey Meyer and his colleagues used PET scans to measure brain inflammation in patients with depression.

They found a 30 percent increase in inflammation in multiple brain regions of those who were suffering depression.

Unfortunately, traditional antidepressant medications have no effect on inflammation.

Future studies will try to determine if medicines with anti-inflammatory effects can treat these symptoms.