The Thyroid Epidemiology, Audit, and Research Study (TEARS): The Natural History of Endogenous Subclinical Hyperthyroidism

Abstract

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. 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. 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. The status of patients was investigated at 2, 5, and 7 yr after diagnosis. 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. Very few SH patients develop frank hyperthyroidism, whereas a much larger proportion revert to normal, and many remain with SH.