PROJECT SUMMARY In most chronic pain syndromes there is a 1.5-2 times higher incidence in females compared to males after puberty, which may in part be due to differences in sex hormone levels and receptors and/or other unknown mechanisms related to sex and/or gender. Gender-affirming hormone therapy (GHT) is routinely used in gender minority (GM) persons' gender-affirming medical care. Masculinizing GHT includes the use of testosterone, whereas feminizing GHT includes estrogen and progesterone, often in conjunction with an anti- androgen. In 2011, The Institute of Medicine identified GM adults as an understudied population in critical need of more health-related research. The GM community includes those who identify as transgender men (TGM), transgender women (TGW), and as non-binary (persons whose gender is not binary). There are only a few studies on how GHT is related to the presence and severity of pain in GM persons. Previous research has shown that GM persons have been reported to have an overall greater prevalence of multiple chronic pain syndromes compared to cisgender persons (persons whose gender corresponds to their sex assigned at birth). Retrospective and cross-sectional studies have identified that GHT may affect the presence and severity of chronic pain in the GM population, with masculinizing GHT being potentially associated with improvements in chronic pain and feminizing GHT with estrogen/progesterone and anti-androgens potentially worsening chronic pain. We hypothesize that the presence of androgenic hormonal influences is protective against the development of a specific chronic pain mechanism, nociplastic pain – pain that appears to be driven by the central nervous system (CNS). The mechanisms that underlie nociplastic pain are not entirely understood, but it is thought that amplified and/or dysregulated pain and sensory signaling within the CNS plays a substantial role. Numerous chronic pain syndromes have been identified as primarily nociplastic and include fibromyalgia, tension headache, and chronic low back pain. These and other pain syndromes often co-occur and are recognized by the NIH as Chronic Overlapping Pain Conditions (COPCs). A large knowledge gap exists and thus presents a unique opportunity to study how GHT may influence the presence and severity of pain in GM persons using a rigorous longitudinal design. Aim 1: To characterize the trajectory of pain and pain-related symptoms in GM adults taking GHT. Aim 2: To perform quantitative sensory testing (QST) and functional brain neuroimaging of GM adults undergoing GHT to examine the mechanism(s) that underlie changes in pain and sensory sensitivity associated with GHT. Aim 3: To perform qualitative studies of how GHT affects gender affirmation, psychological wellbeing, and the experience of pain. We will perform and communicate results of these studies using a community-engaged approach. Data from our work will better enable clinicians to inform GM patients about po...