BACKGROUND: Previous studies have suggested a higher prevalence of thyroid abnormalities in persons with end-stage renal disease. However, little is known regarding the epidemiology of thyroid disorders in persons with less severe kidney dysfunction. METHODS: We used data from the Third National Health and Nutrition Examination Survey to examine the prevalence of hypothyroidism (clinical and subclinical) at different levels of estimated glomerular filtration rate (GFR). We used multivariable logistic regression to evaluate the association between GFR and prevalent hypothyroidism. RESULTS: Among 14,623 adult participants with serum creatinine and thyroid function test results, the mean age was 48.7 years, and 52.6% were women. The prevalence of hypothyroidism increased with lower levels of GFR (in units of mL/min/1.73 m(2)), occurring in 5.4% of subjects with GFR >/=90, 10.9% with GFR 60-89, 20.4% with GFR 45-59, 23.0% with GFR 30-44, and 23.1% with GFR <30 (P < 0.001 for trend). Overall, 56% of hypothyroidism cases were considered subclinical. Compared with GFR >/=90 mL/min/1.73 m(2), reduced GFR was associated with an increased risk of hypothyroidism, after adjusting for age, gender, and race/ethnicity: adjusted odds ratio 1.07 (95% confidence interval: 0.86-1.32) for GFR 60-89, 1.57 (1.11-2.22) for GFR 45-59, 1.81 (1.04-3.16) for GFR 30-44, and 1.97 (0.69-5.61) for GFR <30 mL/min/1.73 m(2) (P= 0.008 for trend). CONCLUSION: Among a nationally representative sample of adults, reduced glomerular filtration rate was associated with a higher prevalence of hypothyroidism, with many subclinical cases. Future studies are needed to determine the potential adverse effects of subclinical and clinical hypothyroidism in persons with chronic kidney disease.