Providers
Our Providers are all Board Certified
At Gastroenterology Specialist, our board certified providers are committed to compassionate, personalized care. By utilizing years of experience and the latest technology, our providers deliver fast, accurate, and comprehensive diagnosis and treatment.
John R. Hood, MD
Dr. John Hood joined Gastroenterology Specialists, Inc. in 1987. He is board certified in gastroenterology and internal medicine and is a Fellow of the American College of Gastroenterology.
Before joining GSI, Dr. Hood has also served as Chairman of the Endoscopy Lab and the Department of Medicine at St. Francis Hospital and as an associate professor of clinical medicine at the University of Oklahoma in Tulsa. He has served as CEO of GSI since 1991.
Board Certification:
American Board of Internal Medicine, Gastroenterology, 1989
American Board of Internal Medicine, Internal Medicine, 1987
Education:
The University of Oklahoma, Oklahoma City, OK – MD, 1982
Oral Roberts University, Tulsa, OK – BS, 1976
Professional Training:
American College of Gastroenterology – Fellow, 1997
University of Oklahoma, Oklahoma City, OK – Gastroenterology Fellowship, 1985-1987
University of Oklahoma Tulsa Medical College, Tulsa, OK – Internal Medicine Residency, 1985
Michael J. Martin, MD
Dr. Michael Martin joined Gastroenterology Specialists, Inc. in 1994. He has been board certified in Gastroenterology since 1994 and was board certified in internal medicine in 1992. He is a member of the Arkansas/Oklahoma Endoscopic Society.
Dr. Martin has special interests in the management of a wide array of gastrointestinal disorders and colon cancer prevention. He has served as Chief of Gastroenterology and Endoscopy at St. Francis Hospital and as an associate professor of clinical medicine at the University of Oklahoma in Tulsa. He has been involved in medical missions and various volunteer efforts.
Board Certification:
American Board of Internal Medicine, Gastroenterology, 1994
American Board of Internal Medicine, Internal Medicine, 1992
Education:
University of Texas Health Science Center, San Antonio, TX – MD, 1989
University of Texas, Austin, TX, – BBA, 1983
Professional Training:
University of Texas Health Science Center, MD Anderson, Houston, TX – Gastroenterology Fellowship, 1992-1994
University of Oklahoma Health Science Center, Tulsa, OK – Internal Medicine Residency, 1989-1992
William K. Briggs, MD
Dr. William Briggs joined Gastroenterology Specialists, Inc. in 2000. He is board certified in gastroenterology and internal medicine.
Before joining GSI, Dr. Briggs was a resident physician at Hillcrest Medical Group and the University of Oklahoma College of Medicine in Tulsa, Oklahoma. He also served as Assistant Clinical Professor in the Department of Internal Medicine at the University of Oklahoma College of Medicine in Tulsa.
Board Certification:
American Board of Internal Medicine, Gastroenterology, 1997
American Board of Internal Medicine, Internal Medicine, 1995
Education:
University of Oklahoma College of Medicine, Tulsa, OK – MD, 1990
University of Oklahoma, Norman, OK – BS, 1984
Professional Training:
Louisiana State University, Baton Rouge, LA – Gastroenterology Fellowship, 1994-1996
University of Southern California, Downey, CA – Hepatology Fellowship, 1993-1994
University of Oklahoma College of Medicine, Tulsa, OK – Internal Medicine Residency, 1993
Jeffrey L. Bigler, MD
Dr. Jeffrey Bigler joined Gastroenterology Specialists, Inc. in 2003. He is board certified in gastroenterology and internal medicine and is a member of the American College of Gastroenterology and the American Society for Gastrointestinal Endoscopy.
Dr. Bigler is primarily interested in inflammatory bowel disease-Crohn’s disease and ulcerative colitis, and colon cancer screening/prevention.
Board Certification:
American Board of Internal Medicine, Gastroenterology, 2003
American Board of Internal Medicine, Internal Medicine, 2000
Education:
University of Oklahoma College of Medicine, Tulsa, OK – MD, 1997
Southwest Baptist University, Bolivar, MO, – BS, 1993
Professional Training:
University of Missouri-Columbia, Columbia, MO – Gastroenterology Fellowship, 2000-2003
University of Oklahoma college of Medicine, Tulsa, OK – Internal Medicine Residency, 2000
Sheldon C. Berger, DO
Dr. Sheldon Berger joined Gastroenterology Specialists, Inc. in 2007. He is board certified in gastroenterology and internal medicine and is a member of AOA, ACOI, OOA, ASGE, ACG, and TOMS.
Before joining GSI, Dr. Berger was the president of Green Country Gastroenterology. He has practiced at numerous hospitals and is a published author with the Journal of Cellular Physiology.
Board Certification:
American Osteopathic Board of Internal Medicine, Gastroenterology, 1991
American Osteopathic Board of Internal Medicine, Internal Medicine, 1990
Education:
University of Health Sciences, College of Osteopathic Medicine, Kansas City, MO – DO, 1984
University of Kansas, Lawrence, KS – BS, 1978
Professional Training:
Chicago Osteopathic Hospital System, Chicago, Illinois – Gastroenterology and Hepatology Fellowship, 1988-1990
Tulsa Regional Medical Center, Tulsa, OK – Internal Medicine Residency, 1988
Tulsa Regional Medical Center, Tulsa, OK – Internship, 1985
David W. Morris, DO
Dr. David Morris joined Gastroenterology Specialists, Inc. in 2007. He is board certified in gastroenterology and internal medicine, is a fellow of the American College of Osteopathic Internists, and is a member of AOA, ACOI, OOA, and TOMS. He is the medical director of Healthcare Research Consultants and is a constant participant in multidisciplinary clinical medical research.
Dr. Morris is active in local and international medical mission work. Before joining GSI, he was a gastroenterologist with Green Country Gastroenterology, an internal medicine physician with Internal Medicine Consultants, and the associate director of medical education with Tulsa Regional Medical Center.
Board Certification:
American Osteopathic Board of Internal Medicine, Gastroenterology, 1995
American Osteopathic Board of Internal Medicine, Internal Medicine, 1992
Education:
Oklahoma State University, College of Osteopathic Medicine, Tulsa, OK – DO, 1986
Oklahoma State University, Stillwater, OK – BS, 1982
Professional Training:
Chicago College of Osteopathic Medicine – Olympia Fields, IL – Gastroenterology Fellowship, 1992-1994
Tulsa Regional Medical Center, Tulsa, OK – Internal Medicine Residency, 1990
Tulsa Regional Medical Center, Tulsa, OK – Rotating Internship, 1987
Roy L. Thompson, MD
Dr. Roy Thompson joined Gastroenterology Specialists, Inc. in 2009. He is board certified in gastroenterology and internal medicine and is a member of the American Gastroenterology Association and the American Society for Gastrointestinal Endoscopy.
Before joining GSI, Dr. Thompson was a gastroenterology physician with Warren Clinic in Tulsa, Oklahoma.
Board Certification:
American Board of Internal Medicine, Gastroenterology, 2008
American Board of Internal Medicine, Internal Medicine, 2004
Education:
University of Oklahoma, Oklahoma City, OK – MD, 2001
University of Oklahoma, Norman, OK – BS, 1997
Professional Training:
University of Tennessee, Memphis, TN – Gastroenterology Fellowship, 2005-2008
University of Tennessee, Memphis, TN – Internal Medicine Chief Resident, 2005
University of Tennessee, Memphis, TN – Internal Medicine Residency, 2007
University of Tennessee, Memphis, TN – Internal Medicine Internship, 2004
Geoffrey A. Fillmore, DO
Dr. Geoffrey Fillmore joined Gastroenterology Specialists, Inc. in 2011. He is board certified in gastroenterology and internal medicine and is a member of the American Society for Gastrointestinal Endoscopy, the American College of Gastroenterology, and the American Gastroenterology Association.
Dr. Fillmore completed extensive training in endoscopic ultrasound (EUS). He currently performs endoscopic ultrasounds in hospital settings. He is a published author with Practical Gastroenterology and Obesity Surgery.
Board Certification:
American Board of Internal Medicine, Gastroenterology, 2011
American Board of Internal Medicine, Internal Medicine, 2007
Education:
Oklahoma State University College of Osteopathic Medicine, Tulsa, OK – DO, 2004
University of Central Oklahoma, Edmond, OK – BS, 2001.
Professional Training:
Scott & White Hospital, Temple, TX – Gastroenterology Fellowship, 2008-2010
Scott & White Hospital, Temple, TX – Internal Medicine Chief Residency, 2008
Scott & White Hospital, Temple, TX – Internal Medicine Residency, 2007
Scott & White Hospital, Temple, TX – Internal Medicine Internship, 2005
Scott E. Hendrickson, DO 
Dr. Scott Hendrickson joined Gastroenterology Specialists, Inc. in 2021. He is board certified in gastroenterology and internal medicine and is a member of the American Society for Gastrointestinal Endoscopy, the American College of Gastroenterology, and the American Gastroenterology Association.
Prior to joining GSI, Dr. Hendrickson served as Chief of Gastroenterology at Cancer Treatment Centers of America, Tulsa and was also faculty and program director for the gastroenterology fellowship program at Oklahoma State University Medical Center. During his gastroenterology fellowship, he completed extensive training in endoscopic ultrasound (EUS) and other advanced endoscopic procedures. His areas of special interest include GI oncology and interventional gastroenterology. Dr. Hendrickson is a published author in Gastrointestinal Endoscopy and other medical journals.
Board Certification:
American Board of Internal Medicine, Gastroenterology, 2014
American Board of Internal Medicine, Internal Medicine, 2002
Education:
Oklahoma State University College of Osteopathic Medicine, Tulsa, OK – DO, 1998
University of Central Oklahoma, Edmond, OK- BS, 1994
Professional Training:
Oklahoma State University- Gastroenterology Fellowship, 2013
Tulsa Regional Medical Center- Internal Medicine Residency, 2002
Tulsa Regional Medical Center- Family Medicine Internship, 1999
Susan VanZandt, FNP-C
Susan VanZandt joined Gastroenterology Specialists in 2017. She is a Family Nurse Practitioner certified by the American Academy of Nurse Practitioners, licensed by the Oklahoma Board of Nursing, and is a member of the AANP and AONP.
Prior to joining GSI, Susan worked as a surgical/trauma intensive care nurse at Saint Francis Hospital and has been a healthcare professional since 2008. She is passionate about gastroenterology and specializes in acute management of gastrointestinal conditions occurring in the hospital setting. She enjoys spending time with her family and traveling.
Board Certification:
Family Nurse Practitioner, AANP, 2017.
Registered Nurse, OBN, 2009.
Education:
Frontier Nursing University, Hyden KY, MSN 2017
Oklahoma Panhandle State University, Goodell OK, BSN 2014
Tulsa Community College, Tulsa OK, ADN 2009
Professional Training:
Access Medical Care, Tulsa OK- Family Medicine , Women’s Health, and Pediatrics
Premier Family Care, Tulsa OK- Family Medicine, Women’s Health, and Pediatrics
OB/GYN Tulsa, Tulsa OK- OB/GYN and Women’s Health
Kristina Rogers, APRN, FNP-C
Kristina Rogers joined Gastroenterology Specialists in 2019. She is a Family Nurse Practitioner certified by the American Academy of Nurse Practitioners, licensed by the Oklahoma Board of Nursing, and is a member of the Association of Oklahoma Nurse Practitioners and the American College of Gastroenterology. She is passionate about gastroenterology and has special interests in managing inflammatory bowel disease, irritable bowel syndrome, gastroesophageal reflux disease and other GI disorders.
Prior to joining GSI, Kristina worked as a family nurse practitioner in a family practice/urgent care clinic. She also has experience working as a labor and delivery registered nurse at Saint Francis Hospital.
Board Certification:
Family Nurse Practitioner, AANP, 2018.
Registered Nurse, OBN, 2013.
Education:
Maryville University, St. Louis, MO, MSN 2017
University of Central Oklahoma, Edmond, OK, BSN 2013
Professional Training:
Access Medical Care, Tulsa OK- Family Medicine, Women’s Health, and Pediatrics
MedRite Health, LLC, Okmulgee, OK – Family medicine, Urgent Care, and Pediatrics
Owasso Pediatric and Adolescent Medicine, Owasso, OK – Pediatrics
Tulsa Women’s Health Care, Tulsa, OK – Obstetrics and gynecology
Lora Beth Horton, APRN, FNP-C
Lora Beth Horton joined Gastroenterology Specialists in 2021. She is a Family Nurse Practitioner certified by the American Academy of Nurse Practitioners, licensed by the Oklahoma Board of Nursing, and is a member of the AANP and AONP.
Prior to joining GSI, Lora Beth worked as a nurse at Ascension Saint John with the Kidney Transplant Center.
Board Certification:
Family Nurse Practitioner, AANP 2020
Registered Nurse, OBN 2011
Education:
University of Oklahoma Health Science Center, Tulsa OK, MSN 2020
Oklahoma City University, Oklahoma City OK, BSN 2011
Oklahoma State University, Stillwater OK, BS 2009
Professional Training:
Warren Clinic, Tulsa OK – Family Medicine
Pediatric Specialist of Tulsa, Tulsa OK – Pediatrics
Warren Clinic Gynecology, Tulsa OK – Women’s Health
Special Delivery Midwifery Care, Tulsa OK – Women’s Health
Courtney Myers, APRN, FNP-C
Courtney Myers joined Gastroenterology Specialists in 2022. She is a Family Nurse Practitioner certified by the American Academy of Nurse Practitioners, licensed by the Oklahoma Board of Nursing, and is a member of the American Academy of Nurse Practitioners, the Association of Oklahoma Nurse Practitioners, the American College of Gastroenterology, and the American Gastroenterology Association.
Prior to joining GSI, Courtney worked as a Family Nurse Practitioner in Family Medicine. She also has experience working in labor and delivery as an RN at Saint Francis Hospital. She enjoys spending time with her family, staying active, and scuba diving.
Board Certification:
Family Nurse Practitioner, AANP, 2021
Registered Nurse, OBN, 2016
Education:
Maryville University, MSN 2021
Oklahoma City University, BSN 2015
University of Oklahoma, BS 2013
Professional Training:
Ascension St. John Riverside, Tulsa, OK- Family Medicine
Tulsa Women’s Health Care, Tulsa OK- Women’s Health
Ascension St. John Urgent Care, Sand Springs, OK – Urgent Care
AMO Health Center, Salina, OK- Pediatrics
Utica Park Clinic-Jenks, Tulsa OK – Primary Care
Mary Chelf, 
APRN, FNP-C
Mary Chelf joined Gastroenterology Specialists in 2022. She is a Family Nurse Practitioner certified by the American Academy of Nurse Practitioners, licensed by the Oklahoma Board of Nursing, and is a member of the American Academy of Nurse Practitioners, the Association of Oklahoma Nurse Practitioners, the American College of Gastroenterology, and the American Gastroenterology Association.
Prior to joining GSI, Mary worked as a Family Nurse Practitioner in Primary Care. She also has experience working as a Critical Care Registered Nurse. She enjoys spending time with her family and traveling.
Board Certification:
Family Nurse Practitioner, AANP, 2020.
Registered Nurse, OBN, 2016.
Education:
University of Oklahoma Health Sciences Center, MSN 2020
University of Oklahoma Health Sciences Center, BSN 2016
Oklahoma State University, BS 2015
Professional Training:
Ascension St. John Primary Care, Tulsa OK-Internal Medicine
St. Mary’s Primary Care, Enid OK-Family Practice Primary Care
Pediatric Specialists of Tulsa, Tulsa OK- Pediatrics
Contemporary Women’s Specialists, Tulsa OK- OB/GYN and Women’s Health