Widespread use of improved measurements for serum thyroid hormones and thyroid-stimulating hormone (TSH) has resulted in characterization of the syndromes of subclinical hypothyroidism, characterized by normal free T4 estimate and raised serum TSH, and subclinical hyperthyroidism, in which patients have normal serum free T4 estimate and decreased serum TSH. Therapy for these two disorders in generally recommended but must be individualized according to the patient's general medical condition. Additional studies are needed to assess the benefits of treatment. This report reviews subclinical hypothyroidism and subclinical hyperthyroidism, describing their causes, diagnostic criteria, complications, and indications for treatment. A brief review of testing for thyroid function is presented, and each of the subclinical disorders is compared with the classic syndromes of hypothyroidism and hyperthyroidism.