Objective: To examine the prevalence of thyroid disease and dysfunction including thyroid autoimmunity in Norway. Materials and methods: All inhabitants 20 years and older (94 009) in Nord-Trùndelag were invited to participate in a health survey with a questionnaire and blood samples. Results: The prevalence of former diagnosed hyperthyroidism was 2.5% in females and 0.6% in males, hypothyroidism 4.8% and 0.9%, and goitre 2.9% and 0.4% respectively. In both sexes the prevalence increased with age. In individuals without a history of thyroid disease the median, 2.5 and 97.5 percentiles for TSH (mU/l) were 1.80 and 0.49±5.70 for females and 1.50 and 0.56±4.60 for males. The TSH values increased with age. When excluding individuals with positive thyroid peroxidase antibodies (TPOAb) (> 200 U/ml), the 97.5 percentiles dropped to 3.60 mU/l and 3.40 mU/l respectively. The prevalence of pathological TSH values in females and males were TSH $10 mU/l 0.90% and 0.37%; TSH 4.1±9.9 mU/l 5.1% and 3.7%; and TSH # 0.05 mU/l 0.45% and 0.20% respectively. The prevalence of positive TPOAb (> 200U/ml) was 13.9% in females and 2.8% in males. In females the lowest percentage (7.9%) of positive TPOAb was seen with TSH 0.2± 1.9 mU/l and increased both with lower and higher levels of TSH. The percentage of males with positive TPOAb was lower than in females in all TSH groups except for those with TSH > 10 mU/l (85% TPOAb positive). Conclusions: In spite of a high prevalence of recognised thyroid disease in the population a considerable number of inhabitants have undiagnosed thyroid dysfunction and also positive TPOAb.

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