DTx is one category of digital health, which also includes a broad swathe of new digital products and services designed to address a broader set of stakeholder needs across a healthcare ecosystem that includes payers, providers, Pharma, Medtech and distributors.
Telehealth is the means by which you communicate with healthcare professionals. Think of it as the channel that connects patients to providers. DTx also uses many telehealth technologies, including video consultation, messaging, remote patient monitoring, and so on.