The Hidden Root Causes of Depression Your Workup May Be Missing

Depression can look like a brain problem when it's really a gut, hormone, inflammation, or nutrient problem. Six root causes worth asking your doctor about before adjusting a prescription.

Written and Reviewed By

Dr. Erin Amato, M.D.

Board-Certified Psychiatrist · Medical Director, Montana Psychiatry

Last updated: September 1, 2026

Steve's Story

Steve had struggled since childhood. Now forty-eight, a property manager and father of six, he'd been told he was lazy, told he wasn't living up to his potential. He'd tried multiple medications for ADHD and anxiety over the years, each of them helpful in one dimension and costly in another.

Then he told me something small, a memory from grade school. On mornings before standardized testing, he would wake up early to make himself a good breakfast, wanting to perform well: pancakes. But by the time he got to school, he was doubled over in the bathroom with stomach cramps and mental fog.

That detail was the whole picture, and no one had ever asked about it.

We ran labs. Steve had celiac disease, an autoimmune reaction to gluten. He had been living with an undiagnosed medical condition that had been generating a cascade of symptoms his entire life, and every one of those symptoms had been treated as a psychiatric problem.

Steve's story is not rare. It is common. And it is why, in my practice, I stopped treating depression as if it were only a brain condition.

The Dominant Model, and What It Misses

Since the release of Prozac in 1988, the dominant framework for treating depression, bipolar disorder, and anxiety has been to view them as neurochemical imbalances. The strategy that follows is straightforward: prescribe medication that alters those neurochemicals.

Neurochemical imbalances are real. They are often present. But the imbalances have causes, and those causes are rarely investigated in a standard psychiatric evaluation. When symptoms of depression appear, they are often the signal that something is off somewhere else in the body. The brain is the loudest reporter, but not always the source of the problem.

This is what functional medicine tries to look at. Rather than stopping at "the symptom is depression, therefore we prescribe an antidepressant," a functional approach asks: what is producing the depression in this specific person?

Six Root Causes Worth Investigating

Below are six of the most common answers I see in my clinic. Any one of them, addressed properly, can meaningfully change the course of someone's depression. Several of them together, when they occur together (which is common), can be transformative.

• Inflammation

  Chronic inflammation is linked to a large share of chronic disease, and it is now well-established as a driver of depression, anxiety, brain fog, and memory decline. Recent research suggests that even early childhood trauma leaves an inflammatory fingerprint that can predispose adults to depression that doesn't respond to standard treatment.

  Inflammation isn't always visible from the outside. It can present as low-grade fatigue, joint pain that comes and goes, brain fog, digestive issues, or simply a sense that the body is running hot. Common contributors: chronic stress, poor sleep, diet high in refined sugar and processed foods, undiagnosed food sensitivities, environmental toxins, and unresolved trauma.

  Worth testing: high-sensitivity CRP, ESR, and homocysteine.

• The Gut

  If you were designing a system where mental health depended on digestive health, you would build the human body. Consider the biology:

  • Ninety-five percent of the body's serotonin is produced in the gut, not the brain. Serotonin is the primary regulator of mood, sleep, appetite, and memory.
  • Seventy to eighty percent of the immune system is located in the gut. Immune activation drives inflammation, which reaches the brain.
  • The vagus nerve is a direct communication highway between the gut and the brain, running in both directions.
  • The gut lining normally has "tight junctions" that keep toxins and undigested food particles contained. When those junctions loosen, a condition often called "leaky gut," those particles can reach the bloodstream and trigger neurological symptoms including anxiety, depression, and brain fog.

  Gluten in particular can drive gut inflammation, even in people who don't have full celiac disease. I recommend most of my depression patients trial a gluten elimination for several weeks. Some report a level of mental clarity they haven't felt in years. Others notice no difference and go back to their usual diet. Either result is useful information.

  Worth testing when gut issues are suspected: comprehensive stool analysis, celiac panel, food sensitivity testing.

• Hormones

  Linda was fifty-seven, a college English professor and mother of three, and had struggled with depression for decades. She had tried course after course of medication, had responded partially to TMS, then relapsed. She was fatigued, migraine-prone, and had underactive thyroid.

  We tested her hormones. Estrogen and progesterone were very low, which for a post-menopausal woman was expected but not being treated. She started bioidentical hormone replacement alongside dietary changes. Within weeks: better sleep, reduced anxiety, fewer migraines, clearer thinking, ten pounds gone.

  Hormones are chemical messengers that regulate mood, sleep cycles, appetite, energy, and stress response. They are almost never checked in a standard depression evaluation. But hormonal imbalance frequently drives psychiatric symptoms, especially at key life transitions:

  • For women: puberty, postpartum, perimenopause, menopause. Any of these can be a hormone-driven depression event and often isn't recognized as one.
  • For men: declining testosterone often presents as low motivation, low mood, brain fog, and loss of drive, and is frequently written off as normal aging.
  • For everyone: thyroid dysfunction, cortisol dysregulation, and insulin resistance all impact mood.

  Worth testing: full thyroid panel (TSH, free T3, free T4, thyroid antibodies), sex hormones appropriate to age and sex, morning cortisol, fasting insulin and glucose.

• Nutrient Deficiencies

  The nutrients that support brain function are not exotic. Several are commonly deficient in people with depression:

  • Vitamin D. Widespread deficiency, especially in northern latitudes and in people who spend most of their time indoors. Depression is a well-documented symptom.
  • Vitamin B12 and folate. Both are required for the synthesis of neurotransmitters. Low levels can cause fatigue, cognitive slowing, and mood disturbance.
  • Iron. Iron deficiency, even before it reaches anemia, is a recognized contributor to fatigue and low mood, particularly in women of reproductive age.
  • Omega-3 fatty acids. Essential for brain structure and inflammation control. Most people don't get enough from diet alone.
  • Magnesium. Involved in over three hundred biochemical reactions. Deficiency is associated with anxiety, poor sleep, and depression.

  Worth testing: vitamin D, B12, folate, ferritin, magnesium, and consider omega-3 index.

• Sleep, Specifically Sleep Apnea

  Untreated obstructive sleep apnea is a common and often unrecognized driver of depression. The disrupted sleep, oxygen desaturation, and metabolic effects all impact mood, energy, and cognition. Patients treated for depression with medications while their sleep apnea goes untreated often show only partial response, because the underlying insult continues every night.

  Snoring, morning headaches, unrefreshing sleep, and daytime fatigue are worth mentioning to your doctor even if you don't think you have a sleep problem. A home sleep study is now easy and inexpensive to arrange.

• Trauma

  Trauma, and particularly early childhood trauma, changes how the nervous system regulates itself. It leaves patterns in the body: hyperarousal, dissociation, chronic muscular tension, sleep disturbance, and a nervous system that stays in a state of low-grade threat.

  Standard antidepressants often do not resolve trauma-driven depression. Trauma-focused therapy (EMDR, somatic experiencing, or trauma-focused CBT) alongside careful medical care is often what actually moves the needle. In some cases, TMS or ketamine therapy can also help, particularly when depressive symptoms are severe enough to keep a person from being able to engage in therapy.

What This Means for Your Next Appointment

The purpose of this list is not to send you into a spiral of new things to worry about. It is to widen the lens.

If you have been in treatment for depression for a while and haven't seen the recovery you were hoping for, some or all of the causes above may be contributing. And each of them can be tested and addressed. Depression that has looked untreatable for years often responds well once the actual driver is identified.

A short list of questions worth bringing to your next visit:

• Have I ever had a full thyroid panel? Have my hormones been checked at all?
• What are my vitamin D, B12, iron, and inflammatory markers?
• Should I try an elimination diet to see if a food sensitivity is contributing?
• Could my sleep quality be affecting my mood?
• Has anyone asked me about trauma?

None of this replaces working with a physician. All of it can help you and your physician have a more productive conversation.

From the Book

Think Outside the Bottle

Part II of the book covers functional medicine, gut health, inflammation, and hormonal drivers of depression in detail.

Get the Book

References and Further Reading

• Institute for Functional Medicine. What Is Functional Medicine?
• Miller AH, Raison CL. The role of inflammation in depression: from evolutionary imperative to modern treatment target. Nature Reviews Immunology. 2016.
• Cryan JF, et al. The Microbiota-Gut-Brain Axis. Physiological Reviews. 2019.
• Amato, E. Think Outside the Bottle: How to Recover from Depression When Antidepressants Fail You. Restoration Press, 2024.

This article is educational and is not a substitute for care from your physician. Do not start or stop medications, supplements, or an elimination diet without input from a qualified clinician.