Depression: What Could Be Causing It

The Biological Basis: Neurotransmitters Are Only Part of the Story

Depression was once explained almost entirely by the "chemical imbalance" theory, centered on low serotonin. That model is incomplete. A 2022 umbrella review published in Molecular Psychiatry (N=361 studies) found no consistent evidence that serotonin levels or serotonin activity differ between people with depression and healthy controls [1]. This does not mean SSRIs are useless. It means the mechanism behind depression is broader than one neurotransmitter.

Genetic vulnerability plays a real role. The largest genome-wide association study on major depressive disorder, published in Nature Genetics with over 800,000 participants, identified 178 genetic risk loci tied to synaptic structure, hormone signaling, and immune function [2]. But genes are not destiny. They set a threshold. Environmental and medical triggers push individuals past it. Dr. Andrew Miller, professor of psychiatry at Emory University, has noted: "Depression is best understood as a systemic illness with roots in immune activation, endocrine disruption, and neural circuit dysfunction, not simply a brain disease" [3].

The practical takeaway is straightforward. Any workup for persistent depression should look beyond the brain. Hormone panels, inflammatory markers, metabolic labs, and a medication review all belong in the initial evaluation.

Thyroid Dysfunction: The Most Commonly Missed Medical Cause

Hypothyroidism mimics depression so closely that misdiagnosis happens routinely. Fatigue, weight gain, cognitive slowing, low motivation. The symptom overlap is nearly total. A study in the Journal of Clinical Endocrinology & Metabolism found that even subclinical hypothyroidism (TSH between 4.5 and 10 mIU/L with normal free T4) was associated with a significant increase in depressive symptoms, particularly in women over 40 [4].

The American Thyroid Association recommends checking TSH in all patients presenting with new-onset depression, especially those who have not responded to first-line antidepressants [5]. Free T4 and thyroid peroxidase (TPO) antibodies should be added when TSH is borderline. Hashimoto's thyroiditis, the most common cause of hypothyroidism in iodine-sufficient countries, produces fluctuating hormone levels that can trigger episodic mood disturbances years before TSH rises out of the standard range.

Treatment with levothyroxine in overtly hypothyroid patients often resolves depressive symptoms within 8 to 12 weeks. For subclinical cases, a trial of low-dose levothyroxine (25 to 50 mcg) may be warranted when depression is the dominant complaint and TSH sits above 7 mIU/L [4].

Sex Hormone Imbalances: Testosterone, Estrogen, and Progesterone

Hormones regulate mood through direct action on GABA receptors, serotonin synthesis, and neuroplasticity. When they decline or fluctuate sharply, depressive symptoms follow.

Testosterone in men. The European Male Ageing Study (EMAS), a population-based cohort of 3,369 men aged 40 to 79, found that men with total testosterone below 300 ng/dL had significantly higher scores on the Beck Depression Inventory compared to men with levels above 400 ng/dL [6]. The Testosterone Trials (TTrials), a coordinated set of seven randomized placebo-controlled trials, showed that testosterone gel in men over 65 with low T produced modest but statistically significant improvements in mood over 12 months [7]. TRT is not a standalone antidepressant, but in hypogonadal men with comorbid depression, restoring testosterone to the mid-normal range (450 to 600 ng/dL) frequently reduces symptom severity.

Estrogen and progesterone in women. The Penn Ovarian Aging Study followed 436 women through the menopausal transition and found that the risk of a new depressive episode was 2.5 times higher during perimenopause compared to premenopause, independent of prior depression history [8]. Rapid estradiol fluctuations, not simply low estrogen, appear to be the primary trigger. The 2022 North American Menopause Society (NAMS) position statement acknowledges that hormone therapy can improve depressive symptoms in perimenopausal women, particularly when initiated during the early transition window [9].

Progesterone metabolites, especially allopregnanolone, are potent GABA-A receptor modulators. Their abrupt withdrawal after childbirth or during the luteal phase underlies postpartum depression and severe PMDD. Brexanolone (Zulresso), an IV allopregnanolone analog, received FDA approval in 2019 for postpartum depression based on two key trials showing a 50% or greater reduction in HAM-D scores within 60 hours [10].

Nutrient Deficiencies That Drive Depressive Symptoms

Several micronutrient deficiencies produce or amplify depression through well-characterized biochemical pathways. The three most clinically relevant are vitamin D, B12/folate, and magnesium.

Vitamin D. A meta-analysis of 31 studies (N=25,916) published in the British Journal of Psychiatry found that participants with serum 25(OH)D below 20 ng/mL had 31% higher odds of depression compared to those with sufficient levels [11]. Vitamin D receptors are densely expressed in the hippocampus, prefrontal cortex, and amygdala. While supplementation trials have shown mixed results, correcting frank deficiency (below 20 ng/mL) to a target of 40 to 60 ng/mL is standard practice in integrative psychiatry and carries minimal downside risk.

Vitamin B12 and folate. Both are required cofactors for the synthesis of S-adenosylmethionine (SAMe), the principal methyl donor in monoamine neurotransmitter production. A cross-sectional analysis from NHANES (N=2,791) found that adults in the lowest quartile of serum folate had significantly higher depression prevalence [12]. Individuals with MTHFR C677T polymorphisms may benefit from L-methylfolate (15 mg/day), which the FDA has cleared as an adjunct to SSRIs.

Magnesium. A 2017 randomized trial in PLOS ONE (N=126) found that magnesium chloride supplementation (248 mg elemental Mg/day for 6 weeks) produced a clinically meaningful improvement in PHQ-9 depression scores, comparable in magnitude to typical SSRI response rates [13]. Magnesium modulates NMDA receptor activity and HPA axis reactivity, two pathways implicated in depression.

Chronic Inflammation: The Immune System Connection

Roughly 25 to 30% of patients with depression show elevated peripheral inflammatory markers, including C-reactive protein (CRP), interleukin-6 (IL-6), and tumor necrosis factor-alpha (TNF-alpha) [3]. This "inflammatory subtype" of depression tends to be treatment-resistant to standard SSRIs.

The evidence is bidirectional. Chronic illness drives inflammation, and inflammation drives depression. Patients with rheumatoid arthritis, inflammatory bowel disease, psoriasis, and type 2 diabetes all carry 1.5 to 2x the rate of comorbid depression compared to age-matched controls [14]. Interferon-alpha therapy for hepatitis C historically caused major depression in 20 to 30% of recipients, providing a natural experiment linking cytokine activation to mood disruption [3].

Dr. Charles Raison, professor of psychiatry at the University of Wisconsin-Madison, has stated: "For a subset of depressed patients, targeting inflammation may be more effective than targeting monoamines. The field needs to move toward biomarker-guided treatment selection" [15]. CRP above 3 mg/L, obtained through a standard high-sensitivity assay, is the most accessible screening marker. Anti-inflammatory interventions being studied include celecoxib augmentation, omega-3 fatty acids (EPA at 1 to 2 g/day), and minocycline.

Medications That Can Cause or Worsen Depression

Drug-induced depression is underrecognized. A 2018 cross-sectional study in JAMA analyzing data from 26 to 192 U.S. adults found that 37.2% of adults use at least one prescription medication that lists depression as a potential adverse effect [16]. Polypharmacy with three or more such medications was associated with a 15% prevalence of concurrent depression, compared to 5% among those not using any depressogenic drugs.

Common offenders include:

• Beta-blockers (propranolol, metoprolol), particularly lipophilic formulations that cross the blood-brain barrier
• Corticosteroids (prednisone above 20 mg/day for more than 2 weeks)
• Hormonal contraceptives, especially depot medroxyprogesterone acetate (DMPA). A Danish nationwide cohort study (N=1,061,997 women) found that DMPA users had a relative risk of 2.7 for first antidepressant use compared to non-users [17]
• Proton pump inhibitors used chronically, potentially through magnesium and B12 depletion
• Isotretinoin (Accutane), though the causal relationship remains debated
• Interferons and certain immunomodulators

A medication reconciliation should be part of every depression evaluation. When a temporal link exists between drug initiation and mood change, a supervised taper or switch is the first intervention, not adding an antidepressant on top.

Sleep Disruption and Circadian Misalignment

The relationship between sleep and depression is not simply that depressed people sleep poorly. Chronic insomnia is an independent risk factor for developing depression. A meta-analysis of 21 longitudinal studies (N=34,067) found that non-depressed individuals with insomnia had a two-fold increased risk of developing depression over follow-up periods ranging from 1 to 34 years [18].

Circadian rhythm disruption compounds the effect. Shift workers, individuals with delayed sleep phase disorder, and those exposed to excessive evening blue light show higher rates of depressive symptoms. Melatonin secretion, cortisol rhythm, and core body temperature cycling all influence monoamine function.

Cognitive behavioral therapy for insomnia (CBT-I) has demonstrated antidepressant effects that persist after the insomnia improves. A 2019 trial in JAMA Psychiatry (N=1,149) found that digital CBT-I reduced both insomnia and depression severity at 10 weeks, with sustained benefits at 1 year [19]. Sleep should be treated as a primary target, not a secondary symptom.

Gut-Brain Axis and the Microbiome

The intestinal microbiome communicates with the central nervous system through the vagus nerve, short-chain fatty acid production, tryptophan metabolism, and direct immune signaling. A 2019 population-level study in Nature Microbiology (N=1,054) identified that individuals with depression had consistently depleted levels of Coprococcus and Dialister species, even after controlling for antidepressant use [20].

This field is still maturing. Probiotic supplementation ("psychobiotics") has shown modest benefits in several small randomized trials, but no specific strain or formulation has reached guideline-level evidence. What is actionable now: clinicians should recognize that chronic antibiotic use, persistent GI symptoms, and restrictive diets may contribute to mood disturbance through microbiome-mediated pathways. A high-fiber, polyphenol-rich diet remains the most evidence-supported intervention for microbiome diversity.

Psychological and Social Drivers

Biology does not operate in a vacuum. Adverse childhood experiences (ACEs) are among the strongest predictors of adult depression. The original ACE Study (N=17,337) found a graded dose-response relationship: individuals with four or more ACEs had a 4.6-fold increase in lifetime depression risk compared to those with zero [21].

Chronic psychosocial stress activates the hypothalamic-pituitary-adrenal (HPA) axis, elevating cortisol. Sustained hypercortisolism damages hippocampal neurons, impairs neurogenesis, and dysregulates inflammatory cascades. This is the biological bridge between life events and brain changes. Job loss, caregiving burden, social isolation, and financial strain all activate this pathway.

Effective treatment pairs biological correction (hormones, nutrients, medication adjustment) with evidence-based psychotherapy. CBT and behavioral activation carry NNT values of 4 to 5 for moderate depression, comparable to antidepressants [22]. Neither approach alone addresses the full picture for most patients.

Building a Proper Diagnostic Workup

A comprehensive depression evaluation should go beyond the PHQ-9. The following labs and assessments form a reasonable first-pass panel:

• Thyroid: TSH, free T4, TPO antibodies
• Sex hormones: total and free testosterone (men), estradiol and progesterone (women, timed to cycle day or menopausal status)
• Nutrients: 25(OH)D, B12, folate, RBC magnesium, ferritin
• Inflammatory markers: hs-CRP, CBC with differential
• Metabolic: fasting glucose, HbA1c, comprehensive metabolic panel
• Medication review: audit all current prescriptions for depressogenic potential
• Sleep assessment: validated screening (ISI or STOP-BANG) and sleep diary
• Psychosocial history: ACE screening, current stressors, substance use

This panel catches the majority of reversible or modifiable contributors. A normal result set narrows the differential to primary psychiatric etiology and strengthens the case for first-line antidepressant therapy or psychotherapy. An abnormal result changes the treatment plan entirely.

Checking fasting AM cortisol (drawn before 9:00 AM) adds value when Cushing syndrome or adrenal insufficiency is suspected, but routine cortisol testing in all depressed patients is not currently supported by guidelines.