Objective: To investigate the long term outcomes for patients with endogenous subclinical hyperthyroidism (SH).
Design: Population record-linkage technology was used retrospectively to identify patients with SH and hospital admissions from January 1, 1993 to December 31, 2009.
Patients: All residents over 18 years old with at least two serum TSH measurements below the reference range for at least 4 months apart and normal free T4/total T4 and normal 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 various medical records matched to five comparators from the general population.
Outcome measures: Cardiovascular disease, fracture, dysrhythmia, dementia and cancer based on ICD 10 codes.
Results: Compared to the reference population, SH was associated with an increased risk of non-fatal cardiovascular morbidity, osteoporotic fracture, dysrhythmia and dementia: adjusted hazard ratios (HR) 1.39 (1.221.58), 1.25 (1.041.50), 1.65 (1.262.17) and 1.64 (1.202.25) respectively. When SH patients who developed overt hyperthyroidism during follow-up were excluded, SH patients were associated with an increased risk of cardiovascular morbidity (HR 1.36 (1.191.57)), dysrhythmia (HR 1.39 (1.021.90)) and dementia (HR 1.79 (1.282.51)), but not fracture and cancer.
Conclusion: Patients with endogenous SH might have an increased risk of cardiovascular disease and dysrhythmia. There is an association with fracture and dementia that is not related to TSH concentration. No association was found between SH and cancer.