OBJECTIVE: The aim of this study was to determine the prevalence of undiagnosed and untreated hyperthyroidism among patients with suppressed thyroid-stimulating hormone (TSH). BACKGROUND: Hyperthyroidism can significantly diminish patient quality of life and increase the financial burden on patients and health systems. We hypothesized that many patients with hyperthyroidism remain undiagnosed because physicians fail to recognize and evaluate suppressed TSH as the first indication of disease. METHODS: We reviewed administrative data on 174011 patients with TSH measured at a tertiary referral center between 2011 and 2017 to identify individuals with hyperthyroidism (TSH <0.05 mU/L) and their subsequent outcomes: evaluation (measurement of T4, T3, radioactive iodine (RAI) uptake scan, thyroid-stimulating immunoglobulin, thyroid peroxidase antibodies) diagnosis, referral and treatment. We used Kaplan-Meier methods and multivariable time-related parametric hazard modeling to measure our outcomes. RESULTS: We found 3336 patients with hyperthyroidism. The mean age of our cohort was 52 ± 17 years, with 79% females and 59% whites. Only 1088 patients (33%) received any appropriate evaluation and hyperthyroidism remained undiagnosed in 37% of patients who had the appropriate workup. Among those diagnosed with hyperthyroidism, only 21% were referred for surgery and 34% received RAI. Predictors for hyperthyroidism diagnosis include lower TSH (0.01u/L), younger age, African-American race, private commercial insurance, being seen in an outpatient setting, absence of medical comorbidities, presentation with ophthalmopathy, or weight loss. CONCLUSIONS: Hyperthyroidism is frequently unrecognized and untreated, which can lead to adverse outcomes and increased costs. Improved systems for detection and treatment of hyperthyroidism are needed to address this gap in care.