The term "subclinical thyroid diseases" describes conditions characterized by low or elevated thyroid-stimulating hormone (TSH), normal levels of circulating thyroid hormones (thyroxine and triiodothyronine), and few or no definite clinical signs or symptoms of thyroid dysfunction. Therefore, it is a diagnosis based on laboratory evaluation, not on clinical criteria. Because the risk of subclinical thyroid disease increases with age, the number of cases should increase as population ages. Their importance lies in the possible adverse effects on the heart, lipids, bone mineral density, quality of life and mortality. Although the subclinical thyroid diseases tend to progress to overt disease, TSH levels in some subjects return to the reference range. However there is controversy about the definition, clinical importance, consequences of untreated disease, whom to screen and when to initiate treatment. We reviewed the literature and our recommendations about screening and treatment are based upon the existing evidence and the author's experience.