Subclinical hypothyroidism, subclinical hyperthyroidism and the sick euthyroid syndrome are three conditions that can be effectively assessed using newer tests. Subclinical hypothyroidism is defined as an elevated sensitive thyrotropin level with a normal free thyroxine level. Replacement therapy with levothyroxine is recommended for patients who have a serum thyrotropin level over 20 microU mL (20 mU per L), patients with thyroid antimicrosomal antibodies, or patients with goiter. A diagnosis of subclinical hyperthyroidism is made when a suppressed serum thyrotropin level occurs in association with normal free thyroxine and triiodothyronine levels. The optimal management of these patients is still under investigation. Close follow-up is necessary to monitor for the possible development of overt hyperthyroidism, tachyarrhythmias or osteoporosis. Critically ill patients with the sick euthyroid syndrome have a wide variety of abnormal results on thyroid function testing. These aberrations are most often related to the underlying disease rather than a primary thyroid disorder.