Functional Medicine in San Antonio: A Root-Cause Approach to Chronic Illness

John Kim
Aug 4
13 min read

Updated: Aug 5

Yoon Hang Kim, MD, MPH Board-Certified in Preventive Medicine | Integrative & Functional Medicine Physician www.directintegrativecare.com

Medical Disclaimer: This article is for educational and informational purposes only and does not constitute medical advice, diagnosis, or treatment. The information presented reflects general principles of integrative and functional medicine and is not intended as a substitute for individualized care from a qualified healthcare provider. Always consult with your physician before starting, modifying, or discontinuing any treatment.

Last updated: August 2026 Medically reviewed by: Yoon Hang Kim, MD, MPH — Board-Certified in Preventive Medicine

What Is Functional Medicine — and Why Does It Matter for Chronic Conditions?

If you are living with a chronic health condition — fatigue that does not resolve with rest, persistent pain, autoimmune flares, digestive dysfunction, brain fog, or symptoms that have eluded a clear diagnosis — you have probably experienced the limits of a symptom-first approach. You receive a prescription for what hurts, an antibiotic for what is inflamed, or a reassurance that test results are "normal" even though you do not feel normal.

Functional medicine takes a fundamentally different approach. Rather than asking what medication matches this symptom, it asks why is this symptom happening in this particular person. The goal is to identify and address the upstream drivers of illness — the root causes — rather than managing downstream effects indefinitely.

This distinction is not philosophical. It changes what gets tested, what gets treated, and how long the practitioner spends understanding your history. In functional medicine, the initial evaluation often takes 60 to 90 minutes because the clinical picture that matters is the full picture: genetics, environment, nutrition, sleep architecture, stress physiology, toxic exposures, gut integrity, hormonal balance, and how all of these interact in your specific body.

How Does Functional Medicine Differ from Conventional Care?

Conventional medicine excels at acute illness, emergencies, surgical intervention, and well-defined infectious disease. For these situations, the diagnostic-and-prescribe model is exactly what is needed, and it saves lives every day.

Where conventional care often struggles is with the complex, multisystem, chronic conditions that now account for the majority of healthcare spending and patient suffering in the United States. Conditions like autoimmune disease, chronic fatigue, fibromyalgia, mast cell activation syndrome (MCAS), irritable bowel syndrome, and chronic pain frequently involve dysfunction across multiple organ systems simultaneously. Treating each symptom in isolation — a gastroenterologist for the gut, a rheumatologist for the joints, a neurologist for the headaches — can produce a medication list without producing improvement.

Functional medicine treats the body as an interconnected system. Digestive dysfunction, for example, does not stay confined to the gut. It can drive systemic inflammation, alter neurotransmitter production, disrupt hormonal balance, and impair immune regulation. Addressing the gut may improve mood, energy, skin, and joint pain — not because the gut is a magic organ, but because the underlying biology is genuinely connected. The published literature increasingly supports this systems-level framework: intestinal permeability and microbial dysbiosis have been linked to autoimmune activation, neuroinflammation, and metabolic disruption across dozens of well-designed studies (Fasano, 2020; Clemente et al., 2018).

This approach requires time, partnership, and a willingness — from both clinician and patient — to look beyond the obvious. It is not faster than conventional care. It is often slower, more detailed, and more demanding. But for the right conditions, it produces results that symptom management alone cannot.

Why Does This Matter Especially for Patients in San Antonio?

San Antonio is the seventh-largest city in the United States, with a diverse population that carries a disproportionate burden of chronic metabolic and autoimmune disease. Texas ranks among the highest states for prevalence of type 2 diabetes, obesity, and autoimmune conditions — many of which have root-cause drivers that are addressable through the functional medicine framework: nutritional insufficiency, environmental exposures, chronic stress, gut dysbiosis, and inflammatory diet patterns.

Despite this need, access to physicians trained in both conventional and functional medicine remains limited in much of Texas. Most primary care visits in the state last 15 minutes or fewer — a time frame that is simply inadequate for the kind of complex, multisystem evaluation that chronic illness demands.

Direct Integrative Care was established to address this gap. The practice model is built around unhurried consultations, comprehensive laboratory evaluation, and a treatment philosophy that integrates evidence-based conventional medicine with functional medicine principles — without abandoning the rigor that sound clinical practice requires.

Who Is Dr. Yoon Hang Kim?

Dr. Yoon Hang "John" Kim has been practicing integrative and functional medicine since 1999 — over 25 years of clinical experience focused specifically on the intersection of conventional medicine, evidence-based complementary therapies, and root-cause resolution for complex chronic illness.

His training and credentials reflect a career built deliberately around this intersection:

Board Certification in Preventive Medicine — conferred by the American Board of Preventive Medicine. This is one of 24 medical specialties recognized by the American Board of Medical Specialties (ABMS) and is focused on disease prevention, health promotion, and population-level health strategy. It is a distinct board certification, not a continuing education certificate.

Residential Fellowship under Dr. Andrew Weil — Dr. Kim completed the Integrative Medicine Fellowship at the University of Arizona's Andrew Weil Center for Integrative Medicine as an Osher Fellow. This is the oldest and most widely recognized university-based integrative medicine fellowship in the world, and Dr. Weil is broadly regarded as the founder of the academic integrative medicine movement. The fellowship is a rigorous, 1,000-hour clinical training program — not a weekend seminar.

Institute for Functional Medicine (IFM) Scholar — Dr. Kim was named a recipient of the 2024 Functional Medicine for All Scholarship, awarded by the Institute for Functional Medicine to physicians committed to bringing functional medicine to underserved and rural populations. This reflects IFM's recognition of Dr. Kim's clinical work and commitment to expanding access to root-cause medicine.

Additional Certifications and Training:

Master of Public Health (MPH), San Diego State University
Medical Degree, Medical College of Wisconsin (Class of 1997)
American Board of Medical Acupuncture (UCLA Medical Acupuncture Training Program)
American Board of Integrative and Holistic Medicine
Howard Hughes Research Fellowship in Pharmacology

Leadership and Academic Positions:

Former Medical Director, Integrative Oncology Program, Miami Cancer Institute at Baptist Health South Florida
Former Director, University of Kansas Medical Center Integrative Medicine Program
Founder, Integrative Health Studies Certificate Program, University of West Georgia
Former Chief Wellness Officer, Memorial Hospital

Publications: Dr. Kim is the author of three books — including the LDN Primer and a clinical LDN textbook — and more than 20 peer-reviewed articles in integrative medicine.

These credentials matter for a specific reason. Under Google's YMYL (Your Money or Your Life) quality guidelines, health content is held to the highest standard of expertise and trustworthiness. The difference between health information written by someone with genuine clinical authority and health information written by a content marketer is not just an SEO consideration — it is a patient safety consideration. Dr. Kim's content on this site and on the companion clinical blog at ifmsynergy.com reflects over two decades of direct clinical experience, not secondhand summaries of other people's work.

What Conditions Does Functional Medicine Address?

Functional medicine is not a replacement for all of conventional medicine. It is a framework that is particularly well-suited to conditions where root-cause complexity outstrips the capacity of a single-system, single-drug approach.

The conditions Dr. Kim most commonly evaluates and manages include:

Autoimmune Conditions — including Hashimoto's thyroiditis, rheumatoid arthritis, lupus, psoriasis, multiple sclerosis, and inflammatory bowel disease (Crohn's disease and ulcerative colitis). The functional medicine approach to autoimmunity focuses on identifying the triggers — gut permeability, molecular mimicry, environmental toxicants, chronic infection, and nutritional insufficiency — that drive immune dysregulation.

Chronic Pain and Fibromyalgia — conditions characterized by central sensitization, neuroinflammation, and dysfunction of the body's pain-modulating systems. Conventional pain management often relies on medications that mask symptoms without addressing the underlying neuroinflammatory or neuroimmune drivers. Functional medicine evaluation may identify contributing factors such as small intestinal bacterial overgrowth (SIBO), mold exposure, nutrient depletion, or hormonal imbalance.

Chronic Fatigue Syndrome (ME/CFS) and Long COVID — complex neuroimmune conditions that involve mitochondrial dysfunction, immune dysregulation, autonomic dysfunction, and post-exertional malaise. These conditions require careful clinical management that respects the patient's energy limitations while systematically addressing identifiable contributors.

Mast Cell Activation Syndrome (MCAS) — a condition involving inappropriate activation of mast cells, leading to multisystem symptoms including flushing, gastrointestinal distress, brain fog, tachycardia, and histamine intolerance. MCAS is frequently underdiagnosed and often overlaps with ME/CFS, dysautonomia, and connective tissue disorders.

Hormonal Imbalances — including thyroid dysfunction, adrenal stress patterns, sex hormone imbalances, and metabolic syndrome.

Digestive Disorders — including SIBO, irritable bowel syndrome, inflammatory bowel disease, and food sensitivities.

Mold-Related Illness — chronic inflammatory response syndrome (CIRS) and mycotoxin-related health effects.

What Is Low Dose Naltrexone (LDN) — and Why Is It Relevant to Functional Medicine?

One of the most clinically significant tools in integrative medicine for complex chronic conditions is low dose naltrexone (LDN). This is an area of particular depth and focus in Dr. Kim's practice — and it illustrates why practitioner expertise matters so much in functional medicine.

Naltrexone is an FDA-approved medication used at standard doses (50 mg) to treat opioid and alcohol use disorders. At low doses — typically 0.5 mg to 4.5 mg — it appears to work through a fundamentally different set of mechanisms: modulating glial cell activity and Toll-like receptor 4 (TLR4) signaling in the central nervous system, reducing neuroinflammation, and potentially supporting endogenous opioid tone through a brief, transient blockade of opioid receptors followed by a compensatory upregulation (Younger et al., 2014; PMID: 24526250).

The published evidence for LDN spans multiple conditions. Small controlled trials have demonstrated benefit in Crohn's disease (Smith et al., 2007; PMID: 17222320; Smith et al., 2011; PMID: 21380937). Pilot and crossover studies have shown signals of benefit in fibromyalgia (Younger et al., 2009; PMID: 19453963; Younger et al., 2013; PMID: 23359310), though the largest randomized trial to date — 99 women receiving 6 mg naltrexone vs. placebo — did not find LDN superior to placebo for its primary pain outcome (Due Bruun et al., 2024; PMID: 38258677). In multiple sclerosis, a pilot trial showed improvement in mental-health quality of life (Cree et al., 2010; PMID: 20695007). A 2026 systematic review and meta-analysis in BMJ Open found moderate-to-large favorable effect sizes for fatigue, pain, and sleep in long COVID and ME/CFS — while explicitly rating overall certainty of evidence as low and noting that no randomized controlled trials were identified (Byambasuren et al., 2026).

Why practitioner expertise matters for LDN: LDN is not a simple prescription. Dosing is highly individualized. Some patients — particularly those with MCAS, ME/CFS, or heightened neurological sensitivity — respond to microgram-level doses and cannot tolerate standard titration protocols. Rapid dose escalation is one of the most common causes of avoidable side effects. The timing of doses, the formulation (immediate-release vs. compounded), the interaction with other medications (particularly opioids — a genuine safety issue requiring perioperative planning), and the integration of LDN into a broader functional medicine treatment plan all require clinical judgment that comes from sustained, focused experience.

Dr. Kim leads the LDN Support Group, a community of over 9,000 members, and is the author of the LDN Primer and a clinical LDN textbook. His clinical work with LDN is documented extensively at ifmsynergy.com, and his approach to LDN therapy is described in detail at Best Doctor for LDN Therapy.

What Should You Expect in a Functional Medicine Evaluation?

A first visit with Dr. Kim typically involves an extended consultation — far longer than a conventional office visit — focused on building a comprehensive understanding of your clinical picture.

Detailed Health History — This includes not just your current symptoms and medication list, but your full medical timeline: childhood illnesses, environmental exposures, dietary patterns, sleep quality, stress history, family history, and the chronological sequence in which your symptoms developed. The timeline often reveals connections that are invisible when symptoms are treated as isolated events.

Advanced Laboratory Testing — Functional medicine often uses laboratory panels that go beyond standard bloodwork. Depending on the clinical picture, this may include comprehensive thyroid panels (not just TSH), advanced lipid markers (such as NMR LDL particle number or ApoB/ApoA-I ratio), inflammatory markers (hs-CRP, cytokine panels), gut microbiome analysis, organic acids testing, nutrient levels, hormone panels, mold and mycotoxin testing, or food sensitivity panels. The goal is to identify the measurable upstream drivers, not just confirm or rule out a named diagnosis.

Individualized Treatment Plan — Based on the history and laboratory findings, a plan is developed that may include nutritional optimization, targeted supplementation, stress and sleep interventions, pharmaceutical therapy where indicated (including LDN for appropriate candidates), medical acupuncture, detoxification support, and lifestyle modification. The plan is iterative — it evolves as your body responds and as follow-up testing reveals changes.

Ongoing Partnership — Functional medicine is not a one-visit solution. Complex chronic conditions require ongoing monitoring, titration, and adjustment. Dr. Kim's practice model is built around this reality, with regular follow-up and accessible communication.

How Is Direct Integrative Care Structured?

Direct Integrative Care uses a membership-based direct care model. This means the practice operates independently of insurance-driven time constraints — allowing for the unhurried, comprehensive consultations that functional medicine requires.

The practice serves patients both in San Antonio and through telemedicine across multiple states, including Texas, Florida, Georgia, Illinois, Iowa, and Missouri. This expanded access is particularly important because many patients with complex chronic conditions — especially ME/CFS, MCAS, and Long COVID — have difficulty finding practitioners with specialized expertise locally.

To begin the process:

Virtual Assessment Form: Complete the form here
Secure Contact: Contact Direct Integrative Care
Phone/Text: (210) 981-6106

Frequently Asked Questions

Is functional medicine evidence-based?

Yes — with an important nuance. Functional medicine draws on the same peer-reviewed biomedical literature as conventional medicine. The difference is in the clinical framework: functional medicine organizes that evidence around root-cause identification and systems biology rather than diagnosis-and-prescribe. Some interventions used in functional medicine have robust clinical trial support; others are supported by mechanistic evidence and clinical experience while larger trials are still underway. A responsible functional medicine physician — as with any good clinician — is transparent about the strength of evidence behind each recommendation.

Does Dr. Kim still practice conventional medicine?

Yes. Dr. Kim is a board-certified physician who is fully trained in conventional medicine. Integrative and functional medicine is not a rejection of conventional care — it is an expansion of it. When a conventional pharmaceutical is the best tool for a given situation, it is used. The goal is to have the broadest possible toolkit and to select the approach that addresses the root cause most effectively.

Can I see Dr. Kim if I do not live in San Antonio?

In most cases, yes. Integative & Functional Medicine Practice of Yoon Hang Kim MD offers telemedicine consultations across multiple states. Many patients with complex chronic conditions — particularly those exploring LDN therapy, MCAS management, or ME/CFS evaluation — work with Dr. Kim remotely.

What is the difference between integrative medicine and functional medicine?

These terms overlap significantly but are not identical. Integrative medicine combines conventional medical treatments with evidence-based complementary therapies (such as acupuncture, nutritional medicine, and mind-body practices). Functional medicine is a clinical framework focused on identifying and treating root causes of disease through a systems biology approach. Dr. Kim's training and practice incorporate both — the therapeutic breadth of integrative medicine and the diagnostic depth of functional medicine.

Is LDN appropriate for everyone with a chronic condition?

No. LDN is a tool — sometimes a powerful one — but it is not universally appropriate. It requires careful patient selection, individualized dosing, and integration into a broader treatment plan. Some patients cannot tolerate LDN at any dose. The decision to use LDN should be made in partnership with a clinician who has specific expertise in its prescribing and management. Dr. Kim's approach to LDN is described in detail at ifmsynergy.com.

Summary

Topic	Key Point
What is functional medicine?	A clinical approach that identifies and addresses root causes of chronic illness rather than managing symptoms in isolation
Who is Dr. Yoon Hang Kim?	Board-certified preventive medicine physician with 25+ years of integrative and functional medicine experience; Andrew Weil Fellow; IFM Scholar; author of 3 books and 20+ peer-reviewed articles
What conditions are addressed?	Autoimmune disease, chronic pain, fibromyalgia, ME/CFS, Long COVID, MCAS, hormonal imbalance, digestive disorders, mold illness
What is LDN?	Low dose naltrexone — a repurposed medication used at low doses for neuroinflammation and immune modulation; an area of particular clinical expertise for Dr. Kim
How is care delivered?	Membership-based direct care model with extended consultations; available in-person (San Antonio) and via telemedicine across multiple states
How do I start?	Complete the Virtual Assessment Form or call/text (210) 981-6106

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About Dr. Kim

Dr. Yoon Hang "John" Kim is a board-certified physician in Preventive Medicine with over 25 years of clinical experience in integrative and functional medicine. He completed a residential fellowship as an Osher Fellow at the University of Arizona's Andrew Weil Center for Integrative Medicine and holds additional certifications in medical acupuncture (UCLA) and integrative and holistic medicine. Dr. Kim is an IFM Scholar and recipient of the 2024 Functional Medicine for All Scholarship from the Institute for Functional Medicine. He was also awarded the Howard Hughes Research Fellowship in Pharmacology.

Dr. Kim specializes in low dose naltrexone (LDN), autoimmune conditions, chronic pain, integrative oncology, fibromyalgia, chronic fatigue syndrome (ME/CFS), mast cell activation syndrome (MCAS), and mold-related illness. He is the author of 3 books — including the LDN Primer and a clinical LDN textbook — and more than 20 peer-reviewed articles. He leads the LDN Support Group, a community of over 9,000 members. He has served as Medical Director of the Integrative Oncology Program at Miami Cancer Institute and Director of the University of Kansas Medical Center Integrative Medicine Program.

Professional: www.yoonhangkim.com | Clinical: www.directintegrativecare.com | Blog: www.ifmsynergy.com

References

Younger J, Parkitny L, McLain D. The use of low-dose naltrexone (LDN) as a novel anti-inflammatory treatment for chronic pain. Clin Rheumatol. 2014;33(4):451–459. doi:10.1007/s10067-014-2517-2. PMID: 24526250
Younger J, Mackey S. Fibromyalgia symptoms are reduced by low-dose naltrexone: a pilot study. Pain Med. 2009;10(4):663–672. doi:10.1111/j.1526-4637.2009.00613.x. PMID: 19453963
Younger J, Noor N, McCue R, Mackey S. Low-dose naltrexone for the treatment of fibromyalgia: findings of a small, randomized, double-blind, placebo-controlled, counterbalanced, crossover trial assessing daily pain levels. Arthritis Rheum. 2013;65(2):529–538. doi:10.1002/art.37734. PMID: 23359310
Smith JP, Stock H, Bingaman S, Mauger D, Rogosnitzky M, Zagon IS. Low-dose naltrexone therapy improves active Crohn's disease. Am J Gastroenterol. 2007;102(4):820–828. doi:10.1111/j.1572-0241.2007.01045.x. PMID: 17222320
Smith JP, Bingaman SI, Ruggiero F, et al. Therapy with the opioid antagonist naltrexone promotes mucosal healing in active Crohn's disease: a randomized placebo-controlled trial. Dig Dis Sci. 2011;56(7):2088–2097. doi:10.1007/s10620-011-1653-7. PMID: 21380937
Cree BA, Kornyeyeva E, Goodin DS. Pilot trial of low-dose naltrexone and quality of life in multiple sclerosis. Ann Neurol. 2010;68(2):145–150. doi:10.1002/ana.22006. PMID: 20695007
Due Bruun K, Christensen R, Amris K, et al. Naltrexone 6 mg once daily versus placebo in women with fibromyalgia: a randomised, double-blind, placebo-controlled trial. Lancet Rheumatol. 2024;6(1):e31–e39. doi:10.1016/S2665-9913(23)00278-3. PMID: 38258677
Byambasuren O, Atkins TJ, Baptista S, Glasziou P, Chakraborty S. Effect of low-dose naltrexone for long COVID: a systematic review and meta-analysis. BMJ Open. 2026;16:e111253. doi:10.1136/bmjopen-2025-111253. (Institute for Evidence-Based Healthcare, Bond University; certainty of evidence rated low by the authors.)
Fasano A. All disease begins in the (leaky) gut: role of zonulin-mediated gut permeability in the pathogenesis of some chronic inflammatory diseases. F1000Res. 2020;9:F1000 Faculty Rev-69. doi:10.12688/f1000research.20510.1. PMID: 32051759
Clemente JC, Manasson J, Scher JU. The role of the gut microbiome in systemic inflammatory disease. BMJ. 2018;360:j5145. doi:10.1136/bmj.j5145. PMID: 29311119
Parkitny L, Younger J. Reduced pro-inflammatory cytokines after eight weeks of low-dose naltrexone for fibromyalgia. Biomedicines. 2017;5(2):16. doi:10.3390/biomedicines5020016. PMC5489802

All references were verified against PubMed or publisher records. Citations reflect the strength and limitations of the underlying evidence, including trials with negative as well as positive results.