Elevation of the free thyroxine index in psychiatric patients.

We calculated a free triiodothyronine (FT3) index on 124 patients who exhibited symptoms and signs of hyperthyroidism and elevations of the free thyroxine (FT4) index on initial screening. A thyrotropin-releasing hormone (TRH) test served as the final arbiter of thyroid function if the clinical presentation was not characteristic of hyperthyroidism or if the FT3 index was not elevated. Forty-one of the 124 patients had normal TRH tests and were thus classified as euthyroid. Of these patients with the so-called euthyroid sick syndrome, 23 had psychiatric disorders. In a separate study, we measured a FT4 index on 100 unselected admissions to the psychiatric ward. Of six patients with elevated FT4 values, only one had hyperthyroidism. We conclude that false positive FT4 index elevations occur commonly in psychiatric patients. The mechanism(s) for the FT4 index elevations remain obscure. Despite some limitations, the TRH test is a valuable diagnostic adjunct for diagnosing hyperthyroidism in the mentally ill patient when other tests and serial observations are inconclusive.