Background: Recent studies in the US have found wide variation in telehealth use across medical specialties (e.g., 6.1% in Allergy-Immunology vs. 24.1% in Cardiology). This is a problem-of-interest, because the US lacks a standardized set of telehealth reimbursement policies, which in turn has hindered telehealth use across all specialties. Despite these policy-level constraints experienced by all specialties, some have normalized telehealth use to mainstream practice, while others are just getting started during the pandemic. Despite accelerated telehealth use during COVID-19 through removal of federal coverage restrictions, uncertainties remain regarding future sustainability. This paper conducts a systematic review to identify “specialty-level factors” historically influencing telehealth use, with a view to identifying implications for widespread sustainability in the post-pandemic era.Methods: We conducted a systematic review and narrative synthesis to examine factors historically influencing telehealth use across six medical specialties in the US, including three “lower-using” specialties (Allergy-Immunology, Gastroenterology, Family Medicine) and three “higher-using” specialties (Cardiology, Psychiatry, Radiology). The “macro-meso-micro” framework was used to guide the review. Article searches were conducted on PubMed. The PRISMA checklist was used to guide reporting of literature reviewed. Three reviewers worked to develop a preliminary synthesis, identify eligibility criteria, explore themes in the data, and assess robustness of final synthesis.Results: Fifty-three articles were reviewed across six medical specialties. The review identified 12 factors across 3 layers, including: 1) macro-layer (policy-level, legal, other-structural), 2) meso-layer (specialty-level historical telehealth rationale, hospital-organizational, specialty-society, treatment, technological, research, cultural) and 3) micro-layer (individual-level provider-and-patient-specific) factors. A key finding was that among “higher-using” medical specialties, the specialty societies and hospital organizations in the meso-layer, proactively promoted telehealth use by influencing both macro- and micro-layer factors (e.g., advocating for consistent payment policies and enabling provider practices to be more tech-savvy & patient-centric, respectively).Conclusion: By identifying a comprehensive set of contextual, individual (and interaction) factors influencing telehealth use across six medical specialties, this review addresses a gap and provides a foundation for future research. Importantly, it identifies: 1) strategies for reducing variation in telehealth use across medical specialties, and 2) implications for ensuring widespread sustainability in the post-pandemic era.