Our Providers

Alessandro Testori, MD

Horizon Primary Care Physician and Researcher

Dr. Alessandro Testori is Board Certified in Internal Medicine and has been practicing in Colorado since 2010. Before that Dr. Testori completed a Residency in Internal Medicine at Mercy Hospital in Chicago, IL and a Fellowship in Immunotherapy at Northwestern University. His training during the fellowship included the use of stem cells in medical procedures, and he co-authored several research papers on stem cell transplantation for the treatment of autoimmune disease and critical limb ischemia. He also trained in the immunotherapy of pancreatic cancer, on a clinical trial using the alpha-gal technology developed by NewLink Genetics Corporation, which was one of the early examples of using genetic engineering for creating cancer vaccines for immunotherapy. Dr. Testori holds a Ph.D. In Molecular Biology from the Flinders University of South Australia and continues to have a strong interest in gene therapy and stem cell biology.

Joe Williams, M.D.


Dr. Williams earned his medical degree from the University of Tennessee and was inducted into Alpha Omega Alpha, the medical school honor society. He also holds a Phd in physiology from West Virginia University. Pediatrics has always been his passion and he enjoys working with children experiencing ADHD, allergy and asthma, obesity and other childhood challenges.

Dr. Williams was born in South Carolina, but raised in Georgia so he’ll always be a bit of a country boy. The joy of practicing pediatrics surrounded by the beauty of Colorado was a lifestyle he could not resist, so he settled here in 1993 and plans to stay forever. He is an avid fly fisherman and enjoy all outdoor activities. Gardening is another one of his passions and he considers himself to be a master at grilling.

"I look forward to meeting your children and designing measures to guide them through a healthy childhood"

Janina Czartolomna, MD

Internal Medicine

Dr. Czartolomna is a licensed Internal Medicine Provider who has been practicing in the Thornton area for over 20 years. She spent 13 years as a pulmonologist in Poland before relocating to the U.S. She completed her residency in Internal Medicine at Presbyterian St. Luke’s Medical Center.

She believes that a preventative lifestyle is an important foundation for overall health. In her spare time, she enjoys travelling and hiking at remote locations (off the beaten path).

Mark Flanigan, MD

Family Medicine

Dr. Flanigan is a Colorado native who received his medical degree from St. Louis University, and is a certified Family Medicine provider. He has been practicing for over 35 years, of which 27 have been in Thornton.

In his spare time, he enjoys spending time with his grandchildren, hiking and playing tennis.

Meggan Macias, PA-C


Meggan Macias is a Physician Assistant with an expertise in pediatrics and adolescent medicine. She was a 2002 graduate of the Child Health Associate/Physician Assistant program at the University of Colorado Health Sciences Center.

She has practiced primary pediatrics for the past ten years and has additional training in adolescent gynecology, behavioral medicine and newborn feeding issues.

Meggan and her husband, Gene, have three children together—twins, Geno and Isabel, and a younger son, Dallas. They live in Parker, CO and enjoy traveling, all types of music and sports activities with their children.

Emily Wilson, PA-C

Family Practice & Woman’s Health

Emily Wilson, is a graduate of Arcadia University’s Physician Assistant program. She has been board certified since 2009 and has been practicing in Colorado since that time. She has a particular interest in Adolescent Women’s Health and contraception.

Prior to PA school Emily was a student athletic trainer for the University of Colorado and acted as the primary healthcare provider for all campers and counselors at an all girls sleep-away camp in Vermont.

In her free time, Emily enjoys spending time camping, rafting, and enjoying the Colorado outdoors.

Whitney Harrington, PA-C


Whitney was born and raised in Oklahoma where she graduated summa cum laude from the University of Oklahoma with a Bachelor’s degree in Psychology and minor in Spanish. She then went on to earn her Masters in Physician Assistant Studies from the University of Colorado’s CHA/PA Program in 2010. Soon after graduation she moved to Idaho where she practiced as a Pediatric PA from 2010-2013.

Whitney joined Horizon Pediatrics in 2014 and enjoys building relationships with families in the greater Denver area. Her interests include adolescent medicine, asthma and allergy, and childhood obesity/nutrition. Whitney also speaks Spanish and is eager to serve the Latino community.

Whitney is a member of both the American Academy of Physician Assistants and American Academy of Pediatrics. In her free time she enjoys spending time with family and exploring the great outdoors while hiking, biking, backpacking, skiing and traveling.

Tony Oakes, PA-C


Tony earned his Physician Assistant certification in 1998 at the University of Colorado Health Sciences Center. He received his Bachelor’s degree in Kinesiology from the University of Colorado at Boulder.

Prior to his work at Horizon pediatrics, Tony was a physician assistant for eight years at Child First Pediatrics, where he provided extensive healthcare to children. He also practiced family medicine at the Inner City Health Center for two years as well as pediatrics at Aspen Family Care for 5 years in Highlands Ranch. Tony enjoys all aspects of pediatrics, particularly adolescent and behavioral medicine.

Tony is actively involved at his church, where he also volunteers regularly. He enjoys and competes regularly in trail ultramarathons. Tony resides in Castle Rock where he passionately loves his two children, Benjamin and Gracie.

New Provider Coming Soon!

Bio coming soon!

Abby Bice, PA-C

Bio coming soon!

Dylan Gallacher, PA-C

Dylan was born and raised in Glenwood Springs, CO and received his Bachelor's degree in Spanish and Sociology from Colorado State University in Fort Collins. He joined Teach for America after college, which took him to Pine Ridge Indian Reservation in South Dakota. During his three years teaching, one of the classes he designed focused on health and nutrition, which sparked his interest further in working in healthcare. He received his PA degree from Rosalind Franklin in Illinois and moved back to Denver to live and work.

Dylan works in primary care three days per week, one day per week in allergy / asthma in our GVR location and is trained to oversee infusions. He also works part-time at Denver Neurological Clinic.

When he's not working, Dylan loves playing soccer, snowboarding, exploring the mountains with friends and listening to podcasts.

Dr. Shantell TwoBears M.D.

Shantell TwoBears M.D.


The physicians and staff from Horizon Pediatrics would like to welcome our newest provider Dr. Shantell TwoBears M.D. She received her Medical degree at University of North Dakota where she was born and raised. She has experience in every aspect of family medicine in women’s health and preventative medicine.

On her free time, she enjoys spending time with her 3 dogs and grandson. She also enjoys reading, traveling, and knitting. Dr. TwoBears will be working out of the Thornton location four days a week and offers early and evening appointments for both children and adults.