BACKGROUND Race/ethnicity may be a newly recognized risk factor for Graves' disease. OBJECTIVE The aim of this study was to examine the prevalence of thyrotoxicosis by race/ethnicity in Americans aged 12-49 years using three National Health and Nutritional Examination Surveys (NHANES). METHODS Data were analyzed from 17,939 participants in NHANES III (1988-1994), NHANES 1999-2002, and NHANES 2007-2010 with available thyroid function test results. Thyrotoxicosis was defined as a serum thyrotropin (TSH) of ≤0.1 mIU/L or subjects taking methimazole or propylthiouracil, and overt thyrotoxicosis was defined as high serum thyroxine and a serum TSH of ≤0.1 mIU/L. Logistic regression was performed accounting for the complex sampling design of NHANES, and the results from all three NHANES surveys were combined 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 [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 [CI 0.6-2.4]; I2 for heterogeneity=0% for both). Among 27 patients with overt thyrotoxicosis, the odds ratio was 8.7 [CI 0.7-112.6] for non-Hispanic blacks and 4.6 [CI 0.4-59.3] for Mexican Americans compared with non-Hispanic whites. CONCLUSIONS The 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.