Misleading T3-test values in psychiatric patients treated with orphenadrine

Thyroid laboratory tests were assessed in 74 psychiatric patients without clinical evidence of overt thyroid disorder. In a first screening of 42 patients, 13 were found to have elevated levels of triiodothyronine uptake test (T3-test) by a Sephadex method. Ten of these patients also had an elevated free thyroxine index (fT4-index). Of the 13 patients with increased T3-test values, 10 were on orphenadrine (Disipal) treatment or had recently stopped taking the drug. In a second study, thyroid function tests were compared in 15 pairs of matched psychiatric patients treated with orphenadrine or other anticholinergic drugs. Values of T3-test and free thyroxine index were significantly higher among patients treated with orphenadrine. Finally, orphenadrine was instituted in four patients and withdrawn in six patients. T3-test values and free thyroxine indices increased gradually after orphenadrine treatment had been initiated and decreased gradually when it was withdrawn.