Nonthyroidal illness syndrome: evaluation of thyroid function in sick patients.

Altered thyroid function tests as a consequence of illness have been recognized for many years, yet the cause and clinical implications remains uncertain. The routine testing of thyroid function in hospitalized patients should be discouraged, as the results are less predictive of primary thyroid disease than in ambulatory patients. Clinicians should be aware of the methods used for thyroid function testing, as the effect of illness on thyroid function varies among the different tests. The most commonly used free T4 assays likely are influenced significantly by nonthyroidal illness. Advances in understanding the basic mechanisms of thyroid hormone metabolism and thyroid hormone action have given insights into the changes in thyroid function tests as a consequence of nonthyroidal illness. In the future, thyroid hormone receptor isoform-specific agonists and antagonists may allow for more specific treatment of select patients with nonthyroidal illness syndrome.