OBJECTIVES: To determine if aging modifies the clinical presentation of hyperthyroidism and the signs of thyrotoxicosis in older people.DESIGN: Prospective cohort study.SETTING: A French university hospital.SUBJECTS: Eighty‐four new patients with overt hyperthyroidism confirmed chemically between January 1992 and January 1993. Controls were 68 older euthyroid patients matched to the older hyperthyroid patients.MEASUREMENTS: Comparison of 19 classical signs of hyperthyroidism between 34 older patients (≥70 years; mean age 80.2) and 50 younger patients (≤50 years; mean age 37.4). Older patients were also compared with controls (mean age 81.3).RESULTS: Three signs were found in more than 50% of older patients: tachycardia, fatigue, and weight loss. Seven signs were found significantly less frequently in older patients (P<.001): hyperactive reflexes, increased sweating, heat intolerance, tremor, nervousness, polydipsia, and increased appetite. Only anorexia (32% vs 4%) and atrial fibrillation (35% vs 2%) were more found frequently in older people (P<.001). A goiter was present in 94% of the younger and in 50% of the older patients (P<.001). The mean number of clinical signs found in the older subjects was significantly smaller than the number found in younger patients (6 vs 10.8;P<.001). Comparison with older controls showed three signs that were highly associated with thyrotoxicosis in older people: apathy (Odd ratio (OR): 14.8), tachycardia (OR: 11.2), and weight loss (OR: 8.7).CONCLUSION: This study confirms the paucity of clinical signs of hyperthyroidism in older adults. These results suggest the necessity of routine screening for thyroid disease in this age group.J Am Geriatr Soc 44:50–5