Background Clinicians who recognize functional neurological disorders (FND) may not share that diagnosis with patients. Poor communication delays treatment and contributes to substantial disability in FND. Diagnostic (ICD-10) coding, one form of medical communication, offers an insight into clinicians’ face-to-face communication. Therefore, quantifying the phenomenon of noncoding, and identifying beliefs and practice habits that reduce coding, may suggest routes to improve medical communication in FND. Methods We reviewed all pediatric neurology consultations in our hospital from 2017 to 2020, selecting those in which neurologists explicitly stated an FND-related diagnosis (N = 57). We identified the neurological symptoms and ICD-10 codes assigned for each consultation. In parallel, we reviewed all encounters that utilized FND-related codes to determine whether insurers paid for this care. Finally, we assessed beliefs and practices that influence FND-related coding through a nationwide survey of pediatric neurologists (N = 460). Results After diagnosing FND, neurologists selected FND-related ICD-10 codes in only 22.8% of consultations. 96.2% of neurologists estimated that they would code for non-epileptic seizure when substantiated by electroencephalography; in practice, they coded for 36.7% of such consultations. For other FND manifestations, neurologists coded in only 13.3% of cases. When presented with FND and non-FND scenarios with equal levels of information, neurologists coded for FND 41% less often. The strongest predictor of noncoding was the outdated belief that FND is a diagnosis of exclusion. Coding for FND never resulted in insurance nonpayment. Conclusion Noncoding for FND is common. Most factors that amplify noncoding also hinder face-to-face communication. Research based on ICD-10 coding (eg, prevalence and cost) may underestimate the impact of FND by >fourfold.