Our Providers

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.

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.

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.

Tori Cohen

Tori Cohen PA

Family Medicine 

Tori Cohen graduated summa cum laude from Arizona State University with a Bachelor's degree in Environmental Science. After college, she decided to move to Colorado and work as a phlebotomist at Boulder Community Hospital. She then went on to earn her Master's in Physician Assistant Studies from the University of Colorado's CHA/PA Program. There she learned the importance of preventative medicine and solidified her desire to work in family medicine.

On her free time, she enjoys spending time outdoors with her family and German Shepherd. Some of her favorite things to do include: reading, backpacking, fishing, snowboarding, and traveling.

Jenna Kueker

Jenna Kueker NP

Nurse Practitioner 

Please welcome our new Nurse Practitioner Jenna Kueker. Jenna grew up in Dallas, TX, and earned her Bachelor of Science in Nursing from the University of Texas at Austin in 2011. She worked for many years as a Registered Nurse in the Emergency Department and earned her Master of Science in Nursing to become a Nurse Practitioner in 2018. Jenna moved to Colorado in 2021 and joined Horizon Pediatrics and Primary Care. She speaks Spanish and loves getting to work with patients of all ages.

Outside of work, Jenna enjoys spending time in the mountains with her family, including her golden retriever. She loves biking, skiing, tap dancing, and cookie decorating.


    Erika Vicente MD

     Dr. Vicente grew up in Reno, NV where she graduated summa cum laude from the University of Nevada,           Reno with a Bachelor’s degree in Biochemistry and Molecular Biology. She earned her medical degree at           the University of Nevada and went on to complete her pediatric residency at the University of California Los       Angeles. 

     Dr. Vicente enjoys all aspects of pediatric care, from newborns to adolescents, and is especially                         interested in chronic medical conditions/complex care, obesity prevention, and behavioral health. 
     She and her boyfriend are excited to call Colorado their new home. Outside of work, she enjoys
and spinning, as well as trying out new restaurants.