Free T4 measurement is preferred to the T4 test for thyroid evaluation in lithium treated patients.

Free T4 (fT4) measurement provides a more sensitive test of thyroid function than the commonly used T4. Recent commercial development of fT4 kit assays has made fT4 assessment widely available. Evaluation of the fT4 Amerlex kit in 26 lithium-treated patients illustrates its utility as a sensitive laboratory test of thyroid function in this group of patients. This finding together with fT4's known superiority over T4 in identifying thyroid dysfunction suggests that fT4 should replace T4 estimations in the routine monitoring of thyroid function in lithium-treated patients.