Depression rarely has a single cause. If your girlfriend is going through a depressive episode, it’s likely the result of several factors layering on top of each other: biology, hormones, life stress, sleep, nutrition, and sometimes medical conditions that haven’t been caught yet. Understanding what might be contributing can help you support her more effectively and recognize when professional help would make a real difference.
Hormones Play a Larger Role Than Most People Realize
Women experience depression at roughly twice the rate of men, and a significant part of that gap traces back to hormonal biology. Estrogen directly influences serotonin, the brain chemical most associated with mood stability. It regulates how serotonin is transported and received between brain cells, and when estrogen levels shift, serotonin activity shifts with it. Progesterone, the other major female sex hormone, works through a different system, calming brain activity through the same pathway that anti-anxiety medications target.
These hormones don’t stay steady. They fluctuate across the menstrual cycle, during and after pregnancy, and during the transition to menopause. For some women, those fluctuations are mild. For others, the drop in estrogen and progesterone during the days before a period triggers a severe mood crash. This condition, called PMDD, causes depressive symptoms that are most intense in the four days before menstruation through the first two or three days of bleeding. It’s distinct from general moodiness: women with PMDD can experience hopelessness, irritability, and withdrawal that genuinely disrupts their ability to function, then feel fine a week later.
Pregnancy and the postpartum period bring even more dramatic hormonal shifts. Psychosocial factors like lack of support, a history of abuse, and high perceived stress significantly increase the risk for depression during and after pregnancy. If your girlfriend has recently been pregnant or given birth, hormonal changes combined with sleep deprivation and the stress of new parenthood create a particularly high-risk window.
Medical Conditions That Mimic or Cause Depression
An underactive thyroid is one of the most overlooked causes of depression. The connection is so well established that the American Association of Clinical Endocrinologists recommends considering a thyroid diagnosis in every patient who presents with depression. Hypothyroid patients frequently show depressive symptoms, cognitive difficulty, apathy, and psychomotor slowing, meaning they think, speak, and move more slowly than usual. In severe cases, the symptoms can look identical to melancholic depression.
Both an overactive and underactive thyroid can cause mood problems, and both are generally reversible with treatment. Among patients with mood disorders, 1 to 4 percent have full-blown hypothyroidism, while subclinical hypothyroidism (a milder, harder-to-detect form) shows up in 4 to 40 percent. A simple blood test can check thyroid function. If your girlfriend hasn’t had one recently, it’s worth mentioning to her doctor.
Vitamin D deficiency is another common contributor. In one study, people with depression had average vitamin D levels of about 23.6 ng/mL, which falls into the “insufficient” range (below 30 ng/mL is considered inadequate). Women in the study had slightly lower levels than men. Vitamin D plays a role in brain function, and low levels are consistently linked to depressive symptoms. Deficiencies in B12, folate, zinc, and omega-3 fatty acids have also been connected to depression.
Sleep Problems and Depression Feed Each Other
Sleep disturbance shows up in roughly 90 percent of people with depression. For a long time, poor sleep was considered just a symptom of depression, something that happened because of the mood disorder. That understanding has changed. Longitudinal research, including a study that followed young men for over 30 years, found that insomnia is an independent risk factor for developing depression later. The relationship runs in both directions: depression disrupts sleep, and disrupted sleep makes depression worse.
The overlap between insomnia and depression appears partly genetic. Studies estimate that 56 percent of the genetic factors influencing insomnia in women overlap with those for major depression. On a practical level, this means that if your girlfriend is sleeping poorly, whether she’s struggling to fall asleep, waking up throughout the night, or sleeping far more than usual, it’s not just a side effect. It’s actively deepening the depression. People with both insomnia and depression tend to have more severe symptoms, need longer treatment, and are less likely to fully recover compared to those with depression alone. Persistent insomnia is also the most common leftover symptom after treatment and a strong predictor of relapse.
Life Stress and Past Experiences
Traumatic events experienced during childhood are a known risk factor for depression at any point in life, regardless of gender. But certain types of trauma hit women disproportionately hard. Sexual abuse and assault occur at three to four times the rate in girls compared to boys, and those experiences prime the brain’s stress-response system in ways that increase vulnerability to depression for years afterward. Your girlfriend may be dealing with the long tail of something that happened well before you met her.
Current stressors matter too. Feeling unsupported, juggling competing demands, financial pressure, relationship conflict, grief, or major life transitions can all trigger or worsen a depressive episode. Women also face specific psychosocial pressures, including body dissatisfaction that often begins in adolescence, caregiving responsibilities that fall disproportionately on them, and societal expectations around emotional labor. None of these causes depression on their own, but stacked together, they create the kind of sustained stress that wears down resilience.
How to Support Her Without Making It Worse
The most important thing you can do is validate what she’s feeling without trying to fix it. Depression makes people feel fundamentally alone, and having a partner confirm that their emotional experience makes sense can counteract that isolation. Validation doesn’t mean agreeing that everything is hopeless. It means communicating that her feelings are real and understandable given what she’s going through.
What tends to backfire is minimizing. Phrases like “just think positive,” “other people have it worse,” or “you have so much to be grateful for” are invalidating because they imply her emotional response is disproportionate or wrong. Even well-intentioned problem-solving (“have you tried exercising more?”) can land as criticism when someone is already struggling to get through the day. A better approach: “I can see this is really hard. I’m here, and I’m not going anywhere.”
Beyond emotional support, you can gently encourage activity without pressure. Depression creates a vicious cycle where withdrawal leads to fewer positive experiences, which deepens the depression. Behavioral activation, a core strategy in depression treatment, works by gradually reintroducing small, manageable activities. You don’t need to be her therapist, but you can be her companion. Suggest starting with two or three easy, low-stakes activities you can do together: a short walk, cooking a meal, watching something she used to enjoy. Frame it as something you’d like to do with her rather than something she should do for her mental health. Working as a team and making gradual, sustained progress is the key principle.
Warning Signs That Need Immediate Attention
Most depression is treatable and manageable with the right support. But certain signs indicate the situation has become more urgent. According to the National Institute of Mental Health, you should take it seriously if she talks about wanting to die, expresses feelings of being a burden to others, or mentions great guilt or shame. Behavioral changes are equally important to watch for: withdrawing from friends, saying goodbye to people as if she won’t see them again, giving away important possessions, taking dangerous risks, or increasing her use of alcohol or drugs.
Extreme mood swings also warrant attention, particularly a sudden shift to calm or even cheerfulness after a period of deep depression, which can sometimes indicate someone has made a decision to act on suicidal thoughts. If any of these signs are new or have recently escalated, connecting her with professional help quickly matters. The 988 Suicide and Crisis Lifeline (call or text 988) is available around the clock.