Objective: To define the rates of progression to frank hyperthyroidism and to normal thyroid function for subclinical hyperthyroid patients (SH).
Design: Record-linkage technology was used retrospectively to identify patients with SH in the general population from 1st January 1993 to 31st December 2009.
Patients: All residents over 18 years old with at least two measurements of TSH below the reference range for at least 4 months from baseline and normal free T4/total T4 and total T3 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.
Outcome measures: The status of patients was investigated at 2, 5 and 7 years 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 (63.0%) after diagnosis. Few patients (0.50.7%) developed hyperthyroidism at 2, 5 and 7 years. The percentage of SH cases reverting to normal increased with time: 17.2% (2 years), 31.5% (5 years) and 35.6% (7 years) and this was more common in SH patients with baseline TSH between 0.1 and 0.4 mU/l.
Conclusion: Very few SH patients develop frank hyperthyroidism whilst a much larger proportion revert to normal, and many remain with SH.