The objective of this study was to define the level of treated thyroid dysfunction in a complete and representative population base in an area of sufficient dietary iodine intake. We used record-linkage technology to retrospectively identify subjects treated for hyperthyroidism or hypothyroidism in the general population of Tayside, Scotland from 1 January 1993 to 30 April 1997. Thyroid status was ascertained by record linkage of patient-level datasets containing details of treatments for hyperthyroidism and hypothyroidism. We identified 620 incident cases of hyperthyroidism, an incidence rate of 0.77/1000·yr [95% confidence interval (CI), 0.70-0.84] in females and 0.14/1000·yr (95% CI, 0.12-0.18) in males. There were 3,486 incident cases of diagnosed primary hypothyroidism, an incidence rate of 4.98/1000·yr (95% CI, 4.81-5.17) in females and 0.88/1000·yr (95% CI, 0.80-0.96) in males. For both hyperthyroidism and hypothyroidism, the incidence increased with age, and females were affected two to eight times more than males across the age range. The mid-year point prevalence of all-cause hypothyroidism rose from 2.2% in 1993 to 3.0% in 1996. The level of thyroid dysfunction in Tayside, Scotland is higher than previously reported, and it increased from 1993 to 1996.