We are led by providers who have dedicated their practice to finding the root cause. Their advanced training and commitment set the standard for care at Functional Medicine Institute.
Dr. Holthouse, MD, IFMCP, is board-certified in family and integrative medicine and a Certified Functional Medicine Practitioner. Passionate about delivering proactive, patient-centered care, Dr. Holthouse focuses on addressing root causes rather than merely managing symptoms. Frustrated with the limitations of traditional medicine, he transitioned to a healthcare model that prioritizes prevention, cures, and lasting wellness over simply prescribing medications. Dr. Holthouse is dedicated to empowering patients to achieve optimal health through personalized and holistic approaches.
Nick Garver is a Physican Assistant who studied at A.T. Still University in Mesa, Arizona. His experience involves Cardio-Metabolic Health, Mens health, General wellness. He has been with Functional Medicine of Idaho since 2024.
Amber Warren received her Master’s in Physician Assistant studies from Midwestern University and is a certified functional medicine practitioner. With additional training in environmental medicine, detox, mold illness, thyroid disease, hormonal imbalances, and autoimmunity, she is well-equipped to investigate the body as a whole. Amber started her career in orthopedic surgery but found her passion in rheumatology, where she explored natural healing and the root causes of autoimmune diseases. She incorporates “food as medicine” into her practice, achieving significant improvements in her patients’ health.
Dr. David Musnick brings over 36 years of expertise in Sports and Internal Medicine, including 30 years in Functional Medicine. He specializes in Orthopedic Sports Medicine, Functional Immunology, Homeopathy, and Functional Neurology, developing innovative approaches to diagnosing and treating joint, tendon, muscle, nerve, and brain-related conditions. Renowned for his work in osteoarthritis, cognitive decline, concussions, and long COVID, Dr. Musnick has treated thousands of complex cases and is a respected author and national speaker.
Dr. Holthouse, MD, IFMCP, is board-certified in family and integrative medicine and a Certified Functional Medicine Practitioner. Passionate about delivering proactive, patient-centered care, Dr. Holthouse focuses on addressing root causes rather than merely managing symptoms. Frustrated with the limitations of traditional medicine, he transitioned to a healthcare model that prioritizes prevention, cures, and lasting wellness over simply prescribing medications. Dr. Holthouse is dedicated to empowering patients to achieve optimal health through personalized and holistic approaches.
Dr. Gavin Guard, DMS, PA-C, CLS is a Doctor of Medical Science–trained Physician Associate who specializes in cardiometabolic health, lipid disorders, diabetes, obesity medicine, hormone replacement therapy, and peptide-based therapies. He is a Clinical Lipid Specialist by the American Board of Clinical Lipidology and focuses on helping patients reduce cardiovascular risk while improving energy, body composition, and overall metabolic health.
Dr. Guard completed advanced doctoral training in clinical medicine and has pursued extensive post-graduate education in lipid medicine, metabolic disease, and preventive care. His clinical approach blends evidence-based medical therapy with nutrition, exercise physiology, lifestyle interventions, and targeted hormone and peptide support to address insulin resistance, lipid disorders, heart disease prevention, weight challenges, and hormonal imbalance.
By combining data-driven care with individualized coaching, Dr. Guard helps patients improve energy, body composition, cardiovascular fitness, and long-term resilience—focusing not only on disease prevention, but on sustained vitality and peak health across the lifespan.
Peer-Reviewed Publications
The Relationship between Gastrointestinal Health, Micronutrient Concentrations, and Autoimmunity: A Focus on the Thyroid
Symptoms Originally Attributed to Thyroid Dysfunction Were Instead Caused by Suboptimal Gastrointestinal Health: A Case Series and Literature Review
Blastocystis hominis; Friend or Foe
Amber Warren received her Master’s in Physician Assistant studies from Midwestern University and is a certified functional medicine practitioner. With additional training in environmental medicine, detox, mold illness, thyroid disease, hormonal imbalances, and autoimmunity, she is well-equipped to investigate the body as a whole. Amber started her career in orthopedic surgery but found her passion in rheumatology, where she explored natural healing and the root causes of autoimmune diseases. She incorporates “food as medicine” into her practice, achieving significant improvements in her patients’ health.
Amy Ray is a board-certified Nurse Practitioner with over 20 years of experience in healthcare. She studied at UC Davis and earned her Master’s degree from MGH Institute of Health Professions. Since 2018, she has been part of Functional Medicine of Idaho. Amy specializes in autoimmune diseases, gut health, and hormone balance, providing personalized care to help her patients achieve optimal wellness.
Christa Inzer Castillo is a board-certified Physician Assistant who studied at The College of Idaho and continued her Masters at Touro University. She has over 14 years of experience in family practice, focusing on primary and acute care. She has been with Functional Medicine of Idaho since 2022 and is passionate about root-cause medicine. Christa specializes in family medicine, gut health, and hormones.
Heather is passionate about helping people feel their best physically, mentally, and emotionally. She takes a whole-person approach to care, focusing on identifying the deeper causes behind symptoms and guiding her patients toward balance, vitality, and confidence in their daily lives. For Heather, functional medicine is more than a career; it’s a calling to help others thrive and enjoy life to the fullest.
Jacqueline Griffin is a board-certified Nurse Practitioner who studied at Chamberlain University. She worked in an internal medicine and family practice, as well as in cardio-thoracic surgery. She has been with Functional Medicine of Idaho since 2024. Jacqueline specializes in gut health, hormones, and trauma.
Karen Callagy is board-certified Physician Assistant who studied at Penn State University and then pursued her Masters in PA at Arizona School of Health Sciences. She practiced family medicine until 2014, where she discovered functional medicine. She has her IFM certification and has been with Functional Medicine of Idaho since 2018, prior with Functional & Integrative Medicine of Idaho. Karen specializes in autoimmune diseases, thyroid diseases, and gut health.
Michael Orloff is a board-certified Physician Assistant with over 10 years of medical experience. He earned his Bachelor of Science from California Polytechnic University and his Master of Physician Assistant Studies from Pacific University in Oregon.
Michael began his career in Emergency Medicine, working in both rural critical access hospitals and large trauma centers. During that time, he frequently encountered advanced stages of disease that might have been prevented if addressed earlier. These experiences sparked his passion for prevention, longevity, and helping patients achieve their healthiest selves.
Today, Michael specializes in cardiometabolic health, hormone health, and longevity medicine. His approach is rooted in addressing the underlying causes of illness, with the goal of preventing disease progression and optimizing long-term wellness.
Sergey Kashubin, NP-C, is a board-certified Nurse Practitioner specializing in family practice, hormone management, and weight loss. He earned his nursing degree from Linfield University and his Master of Nursing from Maryville University. During the COVID-19 pandemic, he sought more effective treatment approaches, leading him to study functional medicine through the Institute of Functional Medicine. This shift transformed his practice from symptom management to addressing the root causes of disease with a holistic approach that considers the whole person—spirit, soul, and body.
Shelby L. Stoddard earned her nursing degree from Millersville University and completed her Family Nurse Practitioner training at Idaho State University. With 30 years of nursing experience and the last 10 years in functional medicine, she is dedicated to uncovering the root causes of illness and guiding patients toward lasting wellness. She emphasizes the integration of the mind, brain, nerves, and immune systems as essential for overall well-being, recognizing that sickness and disease can stem from trauma, toxin exposure, and other pathological insults that contribute to cellular damage. Open-minded and solution-oriented, she provides patients with the tools they need to create meaningful, lasting health changes. Shelby has trained with leading experts in functional medicine and has contributed to clinical research on innovative therapies for chronic illness and pain management.
Andrew Baukol, PA-C, is a certified Physician Assistant with a master’s degree from A.T. Still University and a bachelor’s from Winona State University. Originally from Spring Lake Park, MN, Andrew is driven, passionate, and focused on helping patients take control of their health. With a background in orthopedics and over 10,000 surgeries, he now specializes in sports medicine, hormone replacement, men’s health, and overall health optimization. A lymphoma survivor, Andrew is deeply committed to living life to the fullest.
Dawn Tamagni began her nursing career at 21 as a Registered Nurse in the fast-paced environment of emergency medicine, where she spent nearly a decade building a strong clinical foundation. She went on to earn a Master’s in Nursing Education and taught pharmacology, advanced pathophysiology, and critical care at the college level. She later completed a post-graduate Family Nurse Practitioner certification, working in Electrophysiology and Cardiology for approximately 10 years. Most recently, she served as the sole medical provider in a private rural clinic in Shasta County, California, caring for a broad and diverse primary and urgent care population.
Kerry brings over 13 years of experience as a Family Medicine PA and has a strong background in sports medicine. She’s passionate about lifestyle-driven care and loves helping patients take a proactive approach to their health, especially through wellness, nutrition, movement, and other preventative strategies.
Nick Garver is a Physican Assistant who studied at A.T. Still University in Mesa, Arizona. His experience involves Cardio-Metabolic Health, Mens health, General wellness. He has been with Functional Medicine of Idaho since 2024.
Raquel Kacin, MSN, APRN, FNP-C is a board-certified Family Nurse Practitioner with over 30 years of nursing experience and two decades of advanced practice. She earned her Associate of Applied Science in Nursing from Great Basin College, followed by a Bachelor of Science in Nursing in 2001 and a Master of Science in Nursing in Family Practice in 2005 from the University of Nevada, Las Vegas. Raquel is passionate about delivering quality, compassionate, and comprehensive care to patients of all ages. Her background spans emergency and cardiac care, case management, nursing leadership, and education. As a Nurse Practitioner, she has focused on Internal Medicine, Family Practice, Urgent Care, Occupational Medicine, Workman’s Comp, and both Women’s and Men’s Health. Raquel brings a wealth of experience and a strong commitment to patient-centered care to the Idaho community.
Andrea Corliss, MSN, WHNP-BC is a board-certified Women’s Health Nurse Practitioner with a Master of Science in Nursing from Frontier Nursing University and a bachelor’s degree from The Pennsylvania State University. Andrea entered the world of functional medicine driven by a passion for root cause solutions and a deep commitment to truly listening to women’s concerns. She specializes in sexual health and education, hormone replacement therapy, hormone balancing, women’s wellness, and procedures such as contraceptive implants and endometrial biopsies. Andrea empowers women to make sustainable, healthy lifestyle choices.
Jessica Bennet is a board-certified Nurse-Midwife, who studied at the University of Vermont and pursued her Masters at Frontier Nursing University. She discovered her passion for women’s health with an emphasis on hormone imbalance, as well as primary care. She has been with Functional Medicine of Idaho since 2020. Jessica specializes in Women’s Health with an emphasis on general wellness, fertility, and hormone imbalances.
Kristen Santana, WHNP-BC, AGNP-C, MSNCP, MSCP, is a board-certified Women’s Health and Adult-Gerontology Nurse Practitioner with a passion for holistic, patient-centered care. A Boise native, she integrates evidence-based functional medicine with conventional care to address root causes and promote overall well-being. With a mindful and thorough approach, she specializes in women’s health and primary care.
Brian Hauser, MD is a board-certified physician specializing in Functional and Integrative Family Medicine, with a focus on Autism Spectrum Disorder and related conditions. He earned his undergraduate degree from Emory University, his medical degree from Mercer University School of Medicine, and completed his residency at Rome Family Medicine Residency in Georgia. Originally from Hephzibah, GA, Dr. Hauser was drawn to functional medicine by a desire to help patients and families achieve optimal health and wellness.
Dr. Mario Brus obtained his medical and pediatric specialty training at Loma Linda University in Southern California. He focuses on pediatric functional medicine, seeking to identify the underlying causes of chronic conditions in children through laboratory and genetic testing, along with lifestyle and nutrition changes. Dr. Brus emphasizes a conservative use of medications while strongly advocating for dietary and lifestyle modifications that enhance overall health. His goal is to address health issues at their source by partnering with families and prioritizing their children’s well-being.
Dr. Shana Tubach is a board-certified medical doctor who studied at Creighton University, School of Medicine. With over 20 years of experience in pediatrics, ranging from multiple hospital settings. She has been with Functional Medicine of Idaho since 2024 and is passionate about getting to the root-cause of her patient’s symptoms. Dr. Tubach specializes in Pediatrics, focusing on addressing nutritional and emotional factors.
After being a pediatric NP at a children’s hospital in the Emergency department, I wanted to stop practicing in a “one size fits all” medicine structure and begin learning how to fix and most importantly prevent the underlying cause. After having my daughter and realizing what it felt like to be a concerned parent, I wanted a more personal relationship with my families and patients and to help with the rise of chronic pediatric illness. I have a 1 year old daughter and I love hiking and spending time outdoors with her and my husband and two dogs.
Olivia Mangeac is a certified Physician Assistant with a Bachelor of Science in Biology from Northwest Nazarene University and a Master of Medical Science from Arcadia University. She began her career in Urgent Care at St. Alphonsus, where she practiced for five years. During this time, Olivia developed an interest in root cause medicine and chronic illness prevention, leading her to study functional medicine. Passionate about proactive healthcare, she provides preventative and acute care for infants, children, and adolescents.
Marianne Green is a Licensed Registered Dietitian and an Integrative and Functional Nutrition Certified Practitioner who studied at the University of Alabama and continued her education through Daemen College’s Dietetic Internship. She has been serving patients to optimize their health through the mindset of “Food is Medicine”. She has been with Functional Medicine of Idaho since 2020.
Andrea Wilcox, CFNC is a certified Functional Nutritionist through Mind Body Green with a B.A. in English from California State University, Fullerton. Born in Argentina and raised in Southern California, Andrea was drawn to functional medicine through her personal journey of seeking root-cause solutions and supporting the body’s natural ability to heal through nutrition. She specializes in women’s health, family nutrition, digestive health, weight management, and nutrition education. Andrea has taught cooking and nutrition to children, teens, and adults, combining practical strategies with a creative, individualized approach.
