Subclinical hypothyroidism (SH) is defined as an elevated serum TSH level in presence of normal thyroxine level. SH is a biochemical diagnosis, i.e. it is not possible to diagnose this disorder clinically, although some patients may have non-specific complaints. It is a frequent thyroid disorder even among a healthy population. The prevalence of SH is higher in older people, so it is probably reasonable to use age-specific TSH ranges. The common causes of SH are thyroid autoimmunity or treatment for thyroid disease, however, there are numerous other possible reasons (drug use, endocrine and nonendocrine diseases). As SH may be transient, it is recommended to repeat measurement of the thyroid function in order to confirm the diagnosis and to decide if treatment is needed. It is known that SH may have a negative impact on the state of health, but it is not clear if thyroid hormone substitution can eliminate possible risks and improve the results of treatment. The majority of guidelines recommend thyroid hormone substitution in case TSH level is > 10 mU/l.