Objective: To estimate the prevalence of overmedication with levothyroxine in ambulatory elderly patients and to determine if this older patient population is inherently more likely to have suppressed serum TSH concentration. Research design and methods: The medical records of 180 elderly men above the age of 60 years who were on levothyroxine therapy were reviewed. The prevalence of suppressed TSH as measured by a supersensitive assay in 935 ambulatory patients in a Department of Veterans Affairs Medical Center and 214 unselected elderly subjects presenting to a health fair was also evaluated. Concomitant free T4 measurements done with a competitive binding assay kit were available in 381 patients. Results: The overall prevalence of low serum TSH levels among patients on levothyroxine therapy followed up in an academic center is 13,3%. Patients seen in general medicine (8.7%) and geriatric clinic (7.7%) had the lowest rates of overreplacement with thyroid hormone compared to other medical subspecialty clinics (16.7%). There was a significant correlation between levothyroxine dose used and serum free T4, TSH or log TSH values. The prevalence of low TSH in elderly subjects without known history of thyroid disease was 1.7% in the community and 3.1% in patients seen in clinics. The sensitivity of the pituitary thyrotrophs to thyroid hormone was not increased in elderly patients. Conclusions: The prevalence of overmedication with levothyroxine in elderly patients is high, especially in those seen in community clinics. The suppressed serum TSH levels in elderly patients on levothyroxine is the result of overdosage rather than secondary to increased pituitary gland sensitivity to thyroid hormone.