Background: Race/ethnicity may be a newly recognized risk factor for Graves' disease. Objective: To examine prevalence of thyrotoxicosis by race/ethnicity in Americans aged 12-49 years using three National Health And Nutritional Examination Surveys (NHANES). Methods: We analyzed data from 17,939 participants in NHANES III (1988-1994), NHANES 1999-2002, and NHANES 2007-2010 with available thyroid function test results. We defined thyrotoxicosis as a serum thyroid-stimulating hormone (TSH) ≤0.1 mU/L or subjects taking methimazole or propylthiouracil; and overt thyrotoxicosis as high serum thyroxine and serum TSH ≤0.1 mU/L. We performed logistic regression accounting for the complex sampling design of NHANES and combined results from all three NHANES surveys using a random-effects model. Results: There were 75 study participants with point prevalent thyrotoxicosis, representing a pooled prevalence of 0.4% for Americans aged 12-49 years. Prevalent thyrotoxicosis was nearly three times more likely in non-Hispanic black subjects compared with non-Hispanic whites (OR=2.9 [95%CI 1.5-5.7]), while there was no difference between the prevalence of thyrotoxicosis in Mexican Americans compared to non-Hispanic whites (OR=1.2 [95%CI 0.6-2.4]; I2 for heterogeneity=0% for both). Among 27 patients with overt thyrotoxicosis, the OR was 8.7 (95%CI 0.7-112.6) for non-Hispanic blacks and 4.6 (95%CI 0.4-59.3) for Mexican Americans, compared with non-Hispanic whites. Conclusions: Our results suggest there are race/ethnicity differences in the prevalence of thyrotoxicosis. Future studies should address whether these differences are due to heritable factors, environmental exposures, or a combination of both.