This study investigated whether symptoms and findings of hyperthyroidism exist in patients with subclinical hyperthyroidism (SCH) and sought to determine whether hyperthyroidism treatment improves them. Twenty patients (mean age: 36.10 +/- 1.41 years) and 20 healthy controls (mean age: 36.35 +/- 1.50 years) were included in the study. The SCH duration of patients was at least 6 months. Bone mineral density (BNID) was measured in both patients and controls. The patients were randomly divided into 2 groups of 10 patients each. Symptoms and findings of hyperthyroidism were evaluated and BMD, 24 hour ambulatory blood pressure, holter measurements and serum lipids were determined initially in both groups and 6 months after the attainment of euthyroidism in the treatment group (Group 1) and after a 6 month follow-up in the observation group (Group 2). In the patient group, BMD showed a decrease of 1.3% and 3.9% in femur neck and L1-4 vertebra compared with controls, respectively. But there was no difference in BMD between patients and controls. Fatigue, nervousness, over sweating, tachycardia and tremor improved with treatment. The number of patients with fatigue, nervousness, over sweating and tachycardia increased in Group 2 after the observation. There was no difference between initial values and after a 6 month period from observation or on attainment of euthyroidism in the values of BMD, lipids, minimal and maximal heart rate, total number of ventricular and supraventricular beats and heart rate variability. As a result symptoms of hyperthyroidism were found to be increased in SCH but they partly decreased after antithyroid treatment. But no favourable effects of antithyroid treatment on BMD, heart rate and arrhythmia incidence were found in young, premenopausal patients with SCH during the 6 month period.