Thyroid function testing in psychiatric illness: Usefulness and limitations.

The assessment of thyroid function in psychiatric patients may be obscured by several effects of the psychiatric condition on both thyroid hormone and TSH levels. Acute psychiatric decompensation may result in elevation in total T(4) and free T(4) index, and less frequently in hypothyroxinemia. In addition, psychiatric illnesses can cause suppressed TSH levels, blunted TSH response to thyrotropin-releasing hormone (TRH) (particularly in depression), and elevated TSH values that may result in diagnostic errors. Even though mechanisms similar to the ones responsible for thyroid function test changes in other nonthyroidal illness could account for some of these abnormalities, other mechanisms involving dysregulation of hypothalamic-pituitary function seem to play an important role. TRH stimulation testing has also been used for the diagnosis and prognosis of some psychiatric disorders. This test, however, appears to have low sensitivity and specificity and little clinical usefulness for this purpose and may be replaced by basal TSH levels determined by highly sensitive assays. In this review, in addition to discussing the usefulness and limitations of thyroid function tests in the setting of a psychiatric condition, we provide a stepwise approach, using sensitive TSH as a first-line test in the assessment of thyroid function in psychiatric patients.