We assessed the relationship of serum triiodothyronine (t3) level and risk of disease recurrence in men treated for localized prostate cancer. Participants in our study were found through urology and radiation oncology clinics, and all eligible patients were asked to take part. All patients had been initially diagnosed on the basis of rising prostate specific antigen (PSA) or abnormal physical examination. Histological confirmation of diagnosis was obtained for all subjects. Serum (t3) level was determined by chemoluminescent assay with a standard, commercially available instrument (Immulite Diagnostic Products Corporation, Los Angeles, California). Sixty-eight men with prostate cancer were studied. In our treatment protocol, patients are divided into three risk groups: low risk: serum PSA ≤ 10, stage ≤ T2a, or Gleason grade ≤ 6. These patients are treated with a radioactive implant; moderate risk: serum PSA 10-15 or Gleason 7 or stage ≤ T2b. These patients are treated with 3 months of combined hormonal therapy followed by an implant; high risk: Gleason > 7, tumor in seminal vesicle biopsy, serum PSA > 15 or stage T2c or T3. These patients are treated with 3 months combined hormonal therapy, an implant, and after 2 months break 6000 rad external beam radiotherapy. There was a significant increase in serum t3 with risk category (P = 0.011). Turkey’s multiple range B-test showed a significant difference between the t3 levels of the high risk patients, when compared to the t3 levels of the moderate (P = 0.013) and low risk patients (P = 0.041). The range test showed no significant difference between the t3 levels of the moderate and low risk patients (P = 0.897). Because t3 levels may be affected by age, we performed multivariate linear regression, with t3 as the dependent variable. There was a statistically significant (P = 0.035) association of t3 level with risk group, but there was no significant association of t3 with age (P = 0.803). Multivariate linear regression, with t3 as dependent variable, PSA, Gleason score, and stage as independent variables showed a significant overall association of the three independent variables with t3 (P = 0.042), though individually the relationships were not significant. None of the men had a t3 level that was above the normal range for our laboratory (137 ng/dl). Urologists are actively seeking additional biomarkers of prostate cancer aggressiveness. Many prostate cancers are quite indolent and may never cause a problem, but it is impossible to identify such tumors with certainty. Further studies of serum t3 level as a biomarker in prostate cancer might therefore be worthwhile. With more and better biomarkers, many older men might be spared the rigors of radiation therapy and/or surgery and the complications. Also, new prostate cancer therapies might be directed toward inhibiting the mitogenic effects of t3.