Diagnosis of erectile dysfunction (ED) requires anamnestic investigation, being rarely spontaneously declared by patients. ED occurs frequently in diabetes mellitus, and anecdotal evidence suggests that ED occurs in obesity and in hypothyroidism. The aim of this study was to evaluate the prevalence of ED in patients affected by thyroid disorders (hypothyroidism and hyperthyroidism), in comparison with control subjects and with patients at risk for ED, such as patients with obesity and with type II diabetes mellitus, and the role of age. Spontaneous deposition and International Index of Erectile Dysfunction (IIEF)-5 questionnaire were considered for control subjects and for all patients. Spontaneous deposition of ED occurred for three diabetic patients, never for obese patients, thyroid patients and controls, confirming the value of IIEF-5 in detecting ED. ED was more frequent in obese subjects (42%), and in patients affected by thyroid diseases (59%), than in controls (30%), although less frequent than in type II diabetes mellitus (81%). Both below and above the age of 50 years, ED score was worse in thyroid patients than in control subjects, while ED was more frequent in obese patients than in control subjects only below the age of 50 years.