Background Health communication research and guidelines often recommend that medical terminology be avoided when communicating with patients due to their limited understanding of medical terms. However, growing numbers of e‐patients use the Internet to equip themselves with specialized biomedical knowledge that is couched in medical terms, which they then share on participatory media, such as online patient forums. Objective Given possible discrepancies between preconceptions about the kind of language that patients can understand and the terms they may actually know and use, the purpose of this paper was to investigate medical terminology used by patients in online patient forums. Design Using data from online patient–patient communication where patients communicate with each other without expert moderation or intervention, we coded two data samples from two online patient forums dedicated to thyroid issues. Results Previous definitions of medical terms (dichotomized into technical and semi‐technical) proved too rudimentary to encapsulate the types of medical terms the patients used. Therefore, using an inductive approach, we developed an analytical framework consisting of five categories of medical terms: dictionary‐defined medical terms, co‐text‐defined medical terms, medical initialisms, medication brand names and colloquial technical terms. The patients in our data set used many medical terms from all of these categories. Discussion and conclusions Our findings suggest the value of a situated, condition‐specific approach to health literacy that recognizes the vertical kind of knowledge that patients with chronic diseases may have. We make cautious recommendations for clinical practice, arguing for an adaptive approach to medical terminology use with patients.