Although thyroid disease has been associated with other autoimmune conditions, it is not well recognized in systemic lupus erythematosus (SLE) patients. We found that in 332 SLE patients hospitalized during a 5-year period, the overall prevalence of diagnosed thyroid disease (7.5%) was similar to that in other female populations, but the prevalence of diagnosed hypothyroidism (6.6%) was unexpectedly high. There was also a high frequency of abnormal thyroid function test results in 175 SLE patients without diagnosed thyroid disease who underwent laboratory screening. More than 45% of these patients had elevated levels of thyroid-stimulating hormone, 34% had low T3 determinations, and 18% had high antimicrosomal antibody titers. When patients were categorized into "functional groups," some showed evidence of the "euthyroid sick syndrome" (15%), but many more had laboratory test results suggestive of true (5%) or incipient (39%) primary hypothyroidism. We conclude that abnormal thyroid function test results are common in patients with SLE and that hypothyroidism, especially, should be considered when evaluating symptoms and signs in SLE patients.