The link between hormones and depression is still far from being fully understood, but a link certainly seems to exist.
When we are under stress, our bodies ramp up production of a stress hormone called cortisol. One interesting effect of cortisol is that it may to contribute to fat accumulation in the belly [source: University of Alabama at Birmingham]. This could partially explain a link between obesity and depression, because there doesn't appear to otherwise be a link between body mass index (BMI) and depression. Cortisol levels are normally highest in the morning, and taper off throughout the day. Adults -- and children -- whose cortisol levels remained higher than normal in the afternoon and night were found to be concurrently more likely to report symptoms of depression [source: The Endocrine Society].
According to a recent study, the hormone leptin, which is related to the feeling of fullness when we eat, also seems related to depression [source: The Endocrine Society]. Higher levels of leptin, which appears to have antidepressant qualities that aren't fully understood, also reduces anxiety. Women with higher levels of leptin appear to be at less risk of depression than women with lower levels of leptin. Heavier women generally have higher levels of this hormone and skinnier women have lower levels, but the effect leptin has on depression risk is unrelated to percentage of body fat. Low levels of this hormone may also be linked to other mood disorders, including the eating disorder anorexia nervosa.
Children of bipolar parents have four times the risk of developing mood disorders than children of non-bipolar parents, and the link again seems to be cortisol [source: Concordia University]. Children with bipolar parents are naturally more sensitive to cortisol, and respond to both major and minor stressors with ramped-up levels of the stress hormone.
Women are more likely to develop depression than men, especially during pregnancy, after childbirth and following menopause -- times when hormonal fluctuation is high [source: National Institute of Mental Health]. Interestingly, women are also more likely than men to develop disease of the thyroid, which is heavily involved in hormone production [source: Chakraburtty].
Abuse of alcohol has also been linked to depression, and alcohol abuse is also known to cause a multitude of irregularities when it comes to hormone production and function [source: National Institute on Alcohol Abuse and Alcoholism].
While a confluence of factors seem responsible for depression (including genetics, neurotransmitters and life events such as childhood trauma or divorce), researchers are increasingly confident that hormones, too, play an important -- if not fully understood -- role in adult depression.
See the next section for lots more information about hormones and depression.