Objective: For patients with subclinical hyperthyroidism (SH), the objective of the study was to define the rates of progression to frank hyperthyroidism and normal thyroid function.
Design: Record-linkage technology was used retrospectively to identify patients with SH in the general population of Tayside, Scotland, from January 1, 1993, to December 31, 2009.
Patients: All Tayside residents with at least two measurements of TSH below the reference range for at least 4 months from baseline and normal free T(4)/total T(4) and total T(3) concentrations at baseline were included as potential cases. Using a unique patient identifier, data linkage enabled a cohort of SH cases to be identified from prescription, admission, and radioactive iodine treatment records. Cases younger than 18 yr of age were also excluded from the study.
Outcome measures: The status of patients was investigated at 2, 5, and 7 yr after diagnosis.
Results: We identified 2024 cases with SH, a prevalence of 0.63% and an incidence of 29 per 100,000 in 2008. Most SH cases without thyroid treatment remained as SH at 2 (81.8%), 5 (67.5%), and 7 yr (63.0%) after diagnosis. Few patients (0.5-0.7%) developed hyperthyroidism at 2, 5, and 7 yr. The percentage of SH cases reverting to normal increased with time: 17.2% (2 yr), 31.5% (5 yr), and 35.6% (7 yr), and this was more common in SH patients with baseline TSH between 0.1 and 0.4 mU/liter.
Conclusion: Very few SH patients develop frank hyperthyroidism, whereas a much larger proportion revert to normal, and many remain with SH.