In order to assess thyroid function in former opioid addicts undergoing adjunctive naltrexone (NA) p.o. treatment, we studied 24 subjects (BMI +/- SD: 23.3 +/- 3.2 kg/m2) on 50 mg NA p.o. daily for 15 days to 14.5 months continuously. Measurements included thyrotropin (TSH), total thyroxin (TT4), total triiodothyronine (TT3), while the TT3/TT4100 ratio was calculated as a marker of peripheral conversion of T4 to T3. Reverse T3 (rT3) and serum interleukin-6 (IL-6) levels were also measured. Statistical analysis of thyroid parameters among them, of thyroid parameters versus duration of NA use as well as of thyroid parameters versus BMI was done with linear regression. All the subjects received NA well. The thyroid hormone work-up showed that all the subjects on NA were overall euthyroid. Mean +/- SD levels for TSH were 1.59 +/- 0.29 mU/L, TT4: 171.17 +/- 14.07 nmol/L, TT3: 2.01 +/- 0.27 nmol/L, TT3/TT4100: 1.18 +/- 0.19, rT3: 0.26 +/- 0.07 nmol/L and IL-6: 20.3 +/- 36.6 pg/mL. The duration of NA use was positively correlated with TT3 (r = +0.72, p < 0.001) and TT3/TT4 x 100 (r = +0.77, p < 0.001) and negatively, but not statistically significant, with TT4 (r = -0.38, p = 0.065) and with TSH (r = -0.39, p = 0.062). No significant correlations were found between TT3 and BMI, duration of NA use and rT3 and IL-6. Although few subjects were studied, there are indications that the duration of naltrexone may be positively correlated with TT3 and the ratio of T4 to T3 conversion.