Spectral analysis of heart rate variability (HRV) is a non-invasive method suitable for clinical practice but not yet widely ušed. It allows the sympathetic - parasympathetic modulation of the heart rate to be moni-tored. The aim of the study was to investigate whether the HRV in patients with subclinical hypothyroidism is disturbed in the samé way as in patients with manifest hypothyroidism. In has been demonstrated that patients with manifest hypothyroidism show decreased total HRV and a decrease in the parasympathetic influence both when lying down and standing up, and their relative and absolute sympathetic tonus is also increased. All these parameters become normál once normál thyroid function is achieved. Twenty three females with subclinical hypothyroidism (TSH 4, 5-10 mU/1) were examined, 83 % of them had chronic autoimmune thyroiditis. As controls we ušed 23 healthy females with no history of thyroid dise¬ase. When the patients lay down we noted lowered total spectral effort and lowered low frequency compo-nents which correspond with the sympathetic influence. No significant differences in HRV compared with healthy controls were found when the patients were standing up.

        Key words: autonomie regulation, subclinical hypothyroidism, heart rate variability (HRV).