An analysis of the natural course of subclinical hyperthyroidism

Our aim was to evaluate the incidence rate of overt hyperthyroidism in a cohort of patients with subclinical hyperthyroidism and to assess the potential risk factors for the development of overt thyroid hyperfunction.

We performed a retrospective analysis in 75 patients (68 women, mean age 62.2 +/- 14.2 years) with subclinical hyperthyroidism and different grades of serum thyrotropin (TSH) suppression. Incidence rate of overt hyperthyroidism and survival time, ie, time without requiring therapy for overt hyperthyroidism, were studied.

Thirty-four patients (45.3%) developed overt hyperthyroidism and 15 (20.0%) reverted to normal TSH values. The incidence rate of overt hyperthyroidism was 9.69 cases per 100 patient-year in the whole population and 4.12, 7.41, and 29.63 cases per 100 patient-year in subjects with initial TSH values of 0.30 to 0.49, 0.10 to 0.29, and <0.10 mU/L, respectively. Kaplan-Meier analysis of survival time curves showed that the development of overt thyroid hyperfunction was significantly related to the presence of symptoms of hyperthyroidism (P < 0.05) and low (<0.10 mU/L) TSH concentrations (P < 0.001). A stepwise multivariate Cox regression analysis showed that both symptoms and low TSH values were significant factors for progression to overt thyrotoxicosis.

TSH concentration is the most powerful predictor in the outcome of patients with subclinical hyperthyroidism. Our results suggest that patients with values under 0.10 mU/L have the highest probability to develop overt thyroid hyperfunction. In patients with TSH values higher than this value, the risk of progression is notably lower.