Possible inhibitory effect of lithium on peripheral conversion of thyroxine to triiodothyronine: a prospective study.

The effect of lithium on thyroid function was studied in 18 neuroleptic-treated male chronic schizophrenic in-patients. Lithium carbonate was administered for 8 weeks at a dosage giving a mean serum level of 0.79 mmol/l. Blood was obtained just before and after 8 weeks of lithium administration to determine the serum free thyroxine (free T4) levels, free triiodothyronine (free T3) levels and thyrotropin (TSH) levels. Overall, free T4 and TSH levels significantly increased whereas free T3 levels did not change. Two (11%) patients had abnormally increased free T4 levels and abnormally decreased free T3 levels after 8 weeks of lithium administration. These findings suggest that lithium may inhibit the peripheral conversion of free T4 to free T3 in some susceptible patients.