BACKGROUND: A low serum thyrotropin (TSH) concentration is an excellent predictor of hyperthyroidism, either overt or subclinical. Whether statin use influences the ability of a low serum TSH measurement to detect hyperthyroidism has not been evaluated. METHODS: In a cohort of 307 patients with low or undetectable serum TSH concentrations suggestive of hyperthyroidism, we determined whether concurrent statin use influenced the results of radioiodine uptakes and scans. Participants included 29 patients taking a statin medication and 278 who were not taking a statin. Radioiodine uptakes and scans were interpreted by board-certified nuclear medicine physicians. RESULTS: Sixteen of the 29 patients who were taking a statin (55%) had normal radioiodine uptakes and scans despite their low serum TSH. The remaining 13 low-TSH patients who were taking a statin (45%) had abnormal uptakes and scans, most commonly showing diffuse thyroid hyperplasia with increased radioiodine uptake. In contrast, the vast majority of the 278 patients not taking a statin had abnormal uptakes and scans (84%), while only 16% of them had normal uptakes and scans (p < 0.001 vs. those on statins). The age- and sex-adjusted odds ratio of a statin user with a low serum TSH having normal radioiodine studies (as opposed to abnormal studies) was 3.6 (95% CI, 1.6-8.4). CONCLUSION: In patients with a low serum TSH concentration, normal thyroid function and morphology, as assessed by radioiodine studies, were much more common if the patient was taking a statin. Statins may falsely lower the serum TSH without altering thyroid function ("pseudohyperthyroidism") or, alternatively, statins may improve thyroid function in patients with hyperthyroidism.