Importance of subclinical thyroid disease (STh) is now a matter of discussion. Definition of this unit is laboratory: in presence of normal level of thyroxine (T4) TSH value is changed: in lower TSH level the subclinical hyperthyroidism (STx) in increase TSH levels subclinical hypothyroidism (SH) is present. Risk of clinical manifestation is two three times highter in comparison with persons with normal TSH level. Clinical importance STh is still not evaluated definitively. SH caused disturbance of lipid metabolism, elasticity of vessels and endothelial function and therefore increases risk of atherosclerosis. STx causes electrical instability of myocardium with increased risk of arythmias, increases risk of osteoporosis and other changes. Most important are effects of STh in cardiology, reproductive medicine and gynecology. Clinical significance of these effects is not definitively evaluated.