ContextTo study the prevalence of thyroid dysfunction in a very large unselected population. ObjectiveTo determine the prevalence of abnormal thyroid function and evaluate potential modulatory factors. Design and SettingThe Estudio de Atencion Primaria de Navarra, The APNA Study, is a cross-sectional study conducted in northern Spain. It involved 303883 people, of 20years of age and older, who live in the Navarra region. Participants are covered by the public healthcare system and medical records are digitalized. Main Outcome MeasuresThe information was gathered from e-registered data regarding serum thyrotropin (TSH), thyroid hormones, thyroid antibody concentration and clinical context. Measurements were logged (demographic information and potential thyroid function modulatory factors). ResultsSerum TSH (mU/L) normal range was established at 0.7-4.28. At the time of the study, 87% of the Navarra population had a TSH level within the normal range. Mean serum TSH in euthyroid individuals was higher in women (2.15) than in men (1.96) (P<.001) and higher in the obese with body mass index (BMI) 30kg/m(2) (2.12) as compared to the non-obese BMI <30kg/m(2) (2.06) (P<.001). Mean TSH for the entire population was 1.9. The native Spanish population had statistically significantly lower TSH (1.87) than non-native Spanish (2.15) (P<.001). Additionally, we observed that serum TSH levels decreased with age and an increase in the prevalence of hypothyroidism in the elderly and among people with low-income levels. The prevalence of thyroid dysfunction in Navarra was 12.3%. The prevalence of hypothyroidism (or high TSH) in the population was 8.8% (13.3% in women, 4.2% in men), and the prevalence of hyperthyroidism (or low TSH) was 4.3% (5.6% in women, 3.0% in men). ConclusionsNearly 15% of the general population suffers from biochemical thyroid dysfunction. The serum TSH level appears to be influenced by sex, BMI, age, ethnic origin and socio-economic status.