Dynamics of impending overt hypothyroidism: spontaneous evolution in patients before treatment

The syndrome of subclinical hypothyroidism, with its clinical and metabolic consequences, is frequent. Only a minority of these patients will eventual ly become overtly hypothyroid. It was the aim of the present study to analy se the spontaneous evolution over 10 years of 27 patients with subclinical hypothyroidism who became overtly hypothyroid (group A). For comparison, 27 patients remaining subclinically hypothyroid and matched for their TSH con centrations were characterised (group B). In group A, continuous increase o f TSH concentrations was observed over the whole observation period (p = 0. 002; ANOVA). The concentrations of fT4 remained initially stable, and only fell at a late stage in the three years before overt hypothyroidism (p = 0. 0001; ANOVA). The concentrations of total T3 remained normal throughout the observation period. Thyroid reserve was already impaired at the beginning and during the whole study period. In contrast, these thyroid parameters of patients of group B remained unchanged over 10 years, and thyroid reserve remained normal. The pathogenesis of overt hypothyroidism is a graded proce ss. Increasing concentrations of TSH, a decrease of fT4 and impaired thyroi d reserve are predictors of overt thyroid failure.