The study reported here was undertaken to establish the degree to which a person in a preclinical state of hyperthyroidism, with (by definition) euthyroid T3 and T4 levels but suppressed TRH on testing, already exhibits psychological changes and clinical symptoms. Two groups of 20 patients each, with clear clinical and preclinical hyperthyroidism (as defined by laboratory parameters), were studied, as well as a group of 20 controls. The subjects' psychological state of mind was investigated using self-rating scales, including the state-trait-anxiety inventory (STAI), "Befindlichkeits"-Skala (Bf-S'), depression scale (D-S'), and a list of adjectives (EWL-K) with 14 different aspects of affective moods. Cognitive achievements were evaluated using the d2 test. Subjects were examined for somatic symptoms in accordance with Crooks' index of hyperthyroidism. The results clearly showed that typical psychological and somatic changes are already present in preclinical hyperthyroidism, these changes being partly identical with those of definite hyperthyroidism. In both patient groups, a significant increase in anxiety, a sense of not feeling well, and emotional irritability were found, as well as a tendency towards depressiveness, and an increased lack of vitality and activity. Attentiveness and concentration in both patient groups were lower than in the control group. Both patient groups showed the same prevalence of symptoms, such as palpitations, preference of cold over heat, excessive sweating, nervousness, fine digital tremor, and increased heart rate. With regard to the results, the diagnosis "preclinical hyperthyroidism" thus gains importance. Further prospective studies are required to answer the question whether antithyroidal treatment will influence the described psychological and somatic state of patients with preclinical hyperthyroidism.