Mood disorders arise from a combination of biological, genetic, environmental, and medical factors rather than any single cause. Depression and bipolar disorder, the two main categories, share some underlying mechanisms but differ in important ways. Understanding these causes can help make sense of why mood disorders develop and why they vary so much from person to person.
Brain Chemistry and Chemical Messengers
The brain relies on chemical messengers to regulate mood, motivation, sleep, and emotional responses. When the systems that produce or manage these chemicals fall out of balance, mood regulation suffers. Serotonin is one of the most studied messengers in mood disorders. Low serotonin activity, particularly in the front part of the brain responsible for decision-making and impulse control, has been consistently linked to depression and related conditions. Serotonin normally helps put the brakes on impulsive behavior and promotes a sense of emotional stability. When it’s underactive, that brake weakens.
Serotonin and dopamine (the chemical tied to pleasure, reward, and motivation) work in a push-pull relationship. Serotonin discourages risky or impulsive behaviors while dopamine encourages them. When serotonin drops too low, it can cause the dopamine system to become overactive, tipping the balance toward impulsivity, agitation, or the kind of elevated mood seen in manic episodes. This interaction helps explain why mood disorders often come with changes in motivation, energy, and risk-taking behavior, not just sadness.
Inflammation adds another layer to this chemistry. The body’s immune signaling molecules can cross into the brain and disrupt chemical messenger production in several ways. They increase oxidative stress, which damages a key ingredient the brain needs to manufacture serotonin and dopamine. They also divert tryptophan, the raw material for serotonin, into a different chemical pathway that produces compounds toxic to brain cells. This “cytokine hypothesis of depression” explains why people with chronic inflammatory conditions like autoimmune diseases or long-term infections have higher rates of depression. Chronic exposure to these inflammatory signals can alter brain chemistry enough to trigger or sustain depressive episodes.
Structural Changes in the Brain
Mood disorders aren’t just about chemistry. They also involve measurable physical changes in brain structure. Two areas are especially affected: the hippocampus, which processes memory and helps regulate emotions, and the amygdala, which governs fear and threat responses. In people with major depression, the hippocampus is roughly 8% smaller on both sides compared to people without depression. The amygdala shows a 5 to 7% volume reduction, depending on the side.
These aren’t subtle statistical quirks. Specific subregions within the hippocampus, including areas critical for forming new brain cells and for connecting the hippocampus to the rest of the brain, are consistently smaller in people with chronic or recurrent depression. The size of one particular region has even been identified as a predictor of how long a depressive episode will last. Within the amygdala, the nuclei involved in processing social and emotional information show the most pronounced shrinkage. These structural changes likely contribute to the memory problems, difficulty concentrating, and emotional flatness that many people with depression experience.
The Stress Hormone Connection
Your body has a built-in stress response system that connects the brain to the adrenal glands, which sit on top of your kidneys. When you encounter a threat, this system releases cortisol, the primary stress hormone. Normally, cortisol rises, helps you respond to the situation, then falls back to baseline. In mood disorders, this system often gets stuck in the “on” position.
Chronic stress can keep cortisol levels persistently elevated. Over time, the brain’s receptors for cortisol become less sensitive, like turning down the volume on a smoke alarm. The feedback loop that’s supposed to tell the brain “enough cortisol, stand down” stops working properly. Post-mortem studies of people with severe mood disorders have found reduced numbers of these receptors in the brain, confirming the feedback system is physically impaired.
The consequences ripple outward. Sustained cortisol exposure causes atrophy in the hippocampus and amygdala, the same regions that show volume loss in depression. It impairs memory and executive function. The relationship follows an inverted U pattern: both too much and too little cortisol hurt cognitive performance, while moderate levels are ideal. In bipolar disorder specifically, elevated cortisol, disrupted daily cortisol rhythms, and abnormal stress-response activity are commonly observed during both manic and depressive episodes.
Genetics and Family History
Mood disorders run in families, but the inheritance pattern is complex. Major depression has a heritability of roughly 40 to 50%, and that figure may be higher for severe forms. This means that about half of the risk for depression comes from genetic factors, with the other half coming from environment and life experience. Bipolar disorder is even more heritable, with estimates typically ranging from 60 to 85% in twin studies.
No single gene causes a mood disorder. Instead, many genes each contribute a small amount of risk. Some affect how the brain produces or recycles chemical messengers. Others influence how the stress response system functions or how the brain responds to inflammation. What’s inherited isn’t depression or mania itself, but a biological vulnerability. Whether that vulnerability becomes a full disorder depends heavily on what happens in a person’s life.
Childhood Adversity and Life Events
Adverse childhood experiences, including abuse, neglect, household dysfunction, and exposure to violence, create a strong linear relationship with mood disorder risk. The more adverse events a person experiences in childhood, the higher the likelihood of developing depression or other mood disorders in adulthood. In studies of older adults, those who experienced high adversity or childhood abuse were significantly more likely to have had major depression than those with low adversity.
This isn’t just a psychological effect. Early-life stress physically shapes the developing brain, altering the stress response system and making it more reactive for life. A child who grows up in a chaotic or threatening environment may develop a hair-trigger cortisol response that persists into adulthood, priming the brain for mood instability decades later. This is one of the clearest examples of how genes and environment interact: a person with genetic vulnerability who also experiences childhood adversity faces a much higher risk than someone with only one of those factors.
Loneliness and Social Isolation
Loneliness is both a symptom of mood disorders and a cause. Longitudinal studies, which track people over time, have found that individuals who feel lonely are more likely to develop depression or to relapse after recovery. There’s also emerging evidence that loneliness and forced social isolation can precede manic episodes in people with bipolar disorder. Research conducted before and during the COVID-19 pandemic confirmed that loneliness can function as both a consequence and a precursor of persistent depression and bipolar disorder, creating a cycle that’s difficult to break without intervention.
Thyroid Disease and Medical Conditions
Several medical conditions can produce mood symptoms that closely mimic or overlap with mood disorders. Thyroid disease is one of the most common. An underactive thyroid (hypothyroidism) frequently causes depression and unusual fatigue, while an overactive thyroid (hyperthyroidism) can trigger anxiety, nervousness, and irritability. The more severe the thyroid condition, the more pronounced the mood changes tend to be. Thyroid problems typically cause other symptoms too, like weight changes, temperature sensitivity, or heart rate changes, but mood disturbance is sometimes what brings people to a doctor first.
Vitamin D deficiency, hormonal shifts (including those related to menstruation, pregnancy, and menopause), and neurological conditions like Parkinson’s disease and multiple sclerosis can all produce mood symptoms. This is why a thorough medical workup matters when mood symptoms appear for the first time, especially in the absence of clear psychological triggers. Treating the underlying medical condition often resolves or significantly improves mood symptoms.
Alcohol, Drugs, and Substance Use
Substance use and mood disorders frequently coexist, but truly substance-induced mood disorders, where the mood symptoms are caused entirely by alcohol or drug use, are rarer than you might expect. In a large national survey covering about 19.3 million adults with a current mood disorder, only around 200,000 had episodes classified as exclusively substance-induced. Among people with at least one mood or anxiety disorder, just over 7% reported both independent and substance-related episodes in the prior year.
What’s more common is a feedback loop. Alcohol is a depressant that disrupts sleep, alters brain chemistry, and worsens cortisol dysregulation. Stimulants can trigger manic-like episodes or crash into depressive states. Even when substance use isn’t the sole cause, it reliably makes existing mood disorders harder to treat and more likely to recur. The distinction matters for treatment: if mood symptoms persist well beyond the period of substance use, an independent mood disorder is the more likely explanation.