This study was designed to investigate relationships between serum thyroid hormone levels and the severity of posttraumatic stress disorder (PTSD) symptoms in a group of 65 male Vietnam combat veterans who participated as members of cohorts in an elective inpatient treatment program.Thyroid hormone measures included serum free and total triiodothyronine (T3), free and total thyroxine (T4), and thyroxine-binding globulin. To estimate symptom severity, the Clinician-Administered PTSD Scale (CAPS-2), based on DSM-III-R diagnostic criteria for PTSD, was used. Significant positive correlations were observed between free T3, total T3, total T4, and the "hyperarousal" frequency subscale score and the CAPS-2 frequency sum score. Patients with increased thyroid hormone levels and increased hyperarousal symptoms might constitute a clinically significant subtype among patients with PTSD. Alternatively, increased thyroid activity and hyperarousal symptoms may be associated with phase-related characteristics in PTSD. Research strategies for further evaluation of these preliminary findings are discussed.