Abnormalities in thyroid function associated with chronic therapy with methadone.

We studied 145 clinically euthyroid patients on maintenance methadone therapy for narcotic withdrawal, to characterize abnormalities in thyroid-function tests induced by methadone. About a third had increased total thyroxin (T4) and total triiodothyronine (T3) concentrations in serum. The mean concentrations of T3, T4, and thyroxin-binding globulin (TBG) in serum were significantly greater (P less than 0.001 each) in these patients than in a euthyroid control group. There was a corresponding decrease in the T3 uptake (T3U) test, but the free thyroxin index (FTI) failed to correct for the increased TBG concentration in 15.9% of the patients. Individual and mean concentrations of free T3 (FT3) and free T4 (FT4) in serum and results of an "ultrasensitive" test for thyrotropin (TSH-IRMA) were within normal limits and confirmed the euthyroid state. We conclude that prolonged therapy with methadone causes increases in TBG, T3, and T4 in serum. FT3, FT4, and TSH-IRMA estimations are recommended as the diagnostic thyroid tests to use for patients on methadone maintenance therapy.