Subclinical thyrotoxicosis in an outpatient population - predictors of outcome

Individuals with endogenous subclinical thyrotoxicosis (SCT) may subsequently require treatment for overt disease. We aimed to evaluate the frequency of progression to hyperthyroidism and factors influencing this outcome.

This is a retrospective analysis of outcome in 96 consecutive patients (aged 16-91 years) diagnosed with SCT over a 6-year period. Individuals with secondary causes of TSH suppression were excluded. Mean follow-up was 3·8 years. The significance of age, gender, family history of thyrotoxicosis, symptoms at presentation, thyroid nodule(s) on clinical examination, entry TSH level, antithyroid antibody status and (99m) Tc pertechnetate thyroid imaging results on subsequent development of overt thyrotoxicosis was assessed.

Progression to overt thyrotoxicosis was seen in 8% at 1 year, 16% at 2 years, 21% at 3 years and 26% at 5 years. Multivariate analysis determined that diagnosis as determined by scintiscan to be the only independent predictor of outcome (P = 0·003) with the cumulative percentage requiring therapy at 5 years being 9% for subclinical Graves' disease, 21% for multinodular goitre and 61% for the autonomous nodule subgroup.

Progression of SCT to overt hyperthyroidism occurred at a rate of 5-8% per year with disease aetiology, as determined by thyroid scintigraphy, significantly influencing risk of progression.