BACKGROUND: The impact of subclinical hyperthyroidism (sHT) on the cardiovascular system still needs to be elucidated. The aim of the study was to prospectively assess blood pressure (BP), variability in heart rate, and the prevalence of arrhythmias in patients with sHT, both before and after they are restored to the euthyroid state. METHODS: The study group consisted of 44 normotensive patients (37 women, 7 men), aged 22-65 years (mean+/-SD: 45.9+/-11.0) with sHT. Enrolled patients were drawn from 1080 patients referred to our department for treatment of hyperthyroidism. Study patients were treated with radioiodine treatment to restore the euthyroid state. Ambulatory BP monitoring and Holter electrocardiography were performed (i) when sHT was diagnosed and (ii) at least 6 months after they became euthyroid. RESULTS: sHT in comparison to the euthyroid state was associated with higher (109.3+/-7.1 vs. 107.1+/-7.7 mmHg) nocturnal systolic mean BP (p=0.035) and BP load (14.8 vs. 10.2%, p=0.033), mean diastolic BP (66.4+/-6.6 vs. 64.8+/-6.6 mmHg, p=0.047), and mean arterial pressure (80.8+/-43.1 vs. 79.3+/-43.6 mmHg, p=0.049). Moreover, significant changes in both the time and frequency domain measures of heart rate variability (HRV) were observed: decrease of the square root of the mean squared differences of successive NN intervals (rMSSD) (45.68+/-34.1 vs. 65.09+/-50.6 ms, p=0.03) and the low frequency power (LF) (5.71+/-0.99 vs. 6.0+/-1.01 ms(2), p=0.049) as well as increase of QT interval dispersion (58.25+/-28.5 vs. 46.90+/-12.1 ms, p=0.020). This was accompanied by a clinically insignificant increase in the frequency of ventricular extrasystoles (VES) (3.1+/-7.4 vs. 0.6+/-1.2 per hour, p=0.048) and increased mean heart rate (78.4+/-6.8 vs. 76.0+/-8.0 beats/min, p=0.004). Some of the parameters correlated positively with thyroid hormones: nocturnal diastolic BP with free triiodothyronine (FT(3)) (r=0.397, p=0.008), rMSSD with free thyroxine (FT(4)) (r=0.389, p=0.013), and QT interval dispersion with FT(4) (r=0.450, p=0.004). CONCLUSIONS: The study suggests that sHT in comparison to euthyroid status may be associated with a statistically significant but probably clinically insignificant increase of QT interval dispersion, prevalence of VES, elevated nocturnal arterial BP, and changes in HRV. These findings broaden our understanding of the cardiovascular effects of sHT.