The aim of this prospective cross-sectional study was to investigate the hypertrophic effects of endogenous subclinical hyperthyroidism on myocardium and early development of left ventricular hypertrophy (LVH) in essential hypertensive patients accompanied by endogenous subclinical hyperthyroidism. A total of 31 consecutive patients with stage I hypertension were included in the study. Sixteen of them also had endogenous subclinical hyperthyroidism that they were unaware before. The patients and the controls formed out of ten healthy subjects all underwent an investigation of thyroid functions and cardiologic evaluation. The mean wall thickness of the left ventricle in the stage I hypertensive group with endogenous subclinical hyperthyroidism (group I) was significantly increased as compared with both hypertensive patients without thyroid disease (group II) and the control subjects. The mean left ventricle mass was also significantly higher in group I than group II. Both of the patients' groups had an increased prevalence of LVH as compared with the controls. In this study, hypertensive patients with subclinical hyperthyroidism presented more increase in left ventricular mass, suggesting that subclinical hyperthyroidism may contribute to left ventricular hypertrophy forming a natural progression to hypertension. The hypertensive population should always be screened for endogenous subclinical hyperthyroidism, and should be examined for the criteria of left ventricular hypertrophy by echocardiography in early stages.