Hypothyroidism is a common disease sometimes showing no specific symptoms, thus being difficult to diagnose. This study is aimed at estimating the prevalence of hypothyroidism in Lleida by way of thyroid hormone prescriptions and comparing the results with those from other regions. A retrospective observational study. A total of 2,854 individuals undergoing thyroid hormone treatment through Social Insurance prescriptions. Moths of October-December 2001. age, sex and healthcare region. The prevalence of hypothyroidism for 2001 is 8.4 cases per 1000 inhabitants/day (IC 95%: 9.23-7.47). Average age: 60 (DS <:16.6). Age spread: [email protected] among males and [email protected] among females. By age groups: Under age 15:3., [email protected], 15-64 age group, [email protected] and those over age 64, [email protected] The highest figures were found in the rural areas (Seu d'Urgell: [email protected]), and the lowest in urban areas (Example: [email protected], Balafia: [email protected]) CONCLUSIONS: Hypothyroidism under treatment figure is, as in other studies, higher among females and predominant among those over 64 years of age. We however found prevalence values lower than the data from a prior populational study for the same region, indicating a possible underdiagnosing of this disease.