The QKd interval was utilized as a presumptive index of end-organ thyroid hormone effect to test the hypothesis that patients with panic disorder might have abnormal tissue-level responsivity to normal levels of peripherally circulating thyroid hormones. No significant differences in QKd intervals were found between 15 patients with panic disorder (230 +/- 50 msec) and 20 normal controls (224 +/- 29 msec) while drug-free. These findings suggest that patients with panic disorder have normal tissue-level responsivity to thyroid hormone.