The effect of Li on the peripheral turnover of T4 [thyroxine] and T3 [triiodothyronine] was investigated in 14 euthyroid subjects using lymphocytes as target cells. Li caused a dose-dependent rise on T4 turnover, which did not lead to increased T3 production, but no rise in T3 turnover. Patients [3] with Li induced hypothyroidism were studied. Patients with high T4 turnover had high TSH [thyrotropin] values and in these patients Li induced a decrease in T4 turnover. The demonstrated effect of Li in the control group did not cause the low T4 in these patients. In 2 patients defective T4 degradation was demonstrated.