Meet Your Care Team

The people who make it personal.

We left the system to build something better — a practice where the doctor actually knows your name, has time for your questions, and genuinely cares about what happens to you.

Amity Health care team — Costa Mesa and Orange, CA

“We built the practice we always wished existed.”

Dr. Tsay grew up in Orange County and trained inside the same system most patients know — one built on 7-minute appointments, insurance constraints, and constant pressure to move faster.

He founded Amity Health to restore what mattered: a real doctor-patient relationship, built on time, trust, and continuity.

Marshall joined to handle operations and protect the patient experience across both locations. Dr. Yang joined to bring integrative medicine and deeper root-cause thinking to everyday primary care. Cristina joined to help patients address root causes through functional nutrition.

Together, we built the practice we always wished existed — the kind where you feel known, heard, and taken care of.

Our Doctors & Care Team

Meet the team behind your care.

Dr. Trevor J. Tsay

Board-Certified Family Medicine

Costa MesaOrange

Orange County born and raised, Dr. Tsay attended Villa Park High School before earning his Bachelor of Science in Biopsychology at UC Santa Barbara. He completed his medical education at California Northstate University College of Medicine, then began his residency in General Surgery at UC Davis Medical Center in Sacramento — before finding his true calling in Family Medicine through UC Davis's satellite program in Modesto, CA.

Preventive careMen's healthMetabolic healthFamily medicine

Dr. Cindy T. Yang

Board-Certified Family Medicine

Costa Mesa

Hi, I'm Dr. Cindy Yang, and I'm excited to be a part of Amity Health! I believe great healthcare should feel unrushed, personal, and grounded in real partnership.

Women's healthPreventive medicineIntegrative & functional medicineChronic disease management

Cristina Terwilliger

Functional Nutritionist

Costa Mesa

Cristina Terwilliger came to functional nutrition the way the best practitioners do — through personal experience. After witnessing firsthand how targeted nutrition and lifestyle changes could transform her own health, she made it her mission to help others uncover the root causes behind their symptoms rather than simply managing them. At Amity Health, Cristina combines advanced functional testing — including GI-MAP gut microbiome analysis — with individualized, science-based nutrition strategies to address digestion, inflammation, hormone and thyroid balance, stress recovery, and immune optimization.

Gut healthHormone balanceThyroid supportInflammation reductionImmune optimization

Marshall MacDonald

MBA

Co-Founder & CEO

Marshall grew up in the Inland Empire and went on to earn a Bachelor of Science in Environmental Science from UC Santa Barbara and an MBA from UC Riverside. An athlete at heart — surfer, snowboarder, weightlifter, Brazilian Jiu-Jitsu practitioner — Marshall has always treated his health as the foundation that makes everything else possible. That foundation was tested repeatedly.

Why We Practice DPC

Medicine built on trust, not transactions.

Time

Your doctor has time for you. Not 7 minutes — as long as it takes.

Access

Text your doctor directly. Same-day appointments. Real answers, not automated responses.

Transparency

One flat monthly fee. No copays, no surprise bills, no insurance games.

Ready to meet your doctor?

Both locations are accepting new members. Same-day appointments available.

Become a Member View Locations