Differences in hypothyroidism between lithium-free and -treated patients with bipolar disorders

The majority of the previous studies of thyroid abnormalities in bipolar patients was conducted in populations containing various proportions of lithium-treated subjects. In the present study, we sought to determine whether there exist differences in hypothyroid profile between lithium-free and -treated bipolar patients. Bipolar patients never treated with lithium and carbamazepine (n = 78) and those currently in lithium therapy (n = 53) were included in this study. Serum concentrations of total thyroxine (T₄), total triiodothyronine (T₃), and thyroid-stimulating hormone (TSH) were compared between lithium-free and -treated patients. The rate of hypothyroidism in lithium-free patients was significantly lower than those treated with lithium (6.3%-10.8% vs. 28.0%-32.1%). Significant changes in the three thyroid indices indicative of hypothyroidism were consistently associated with longer illness duration in lithium-free manic patients, but with greater severity of mania and more mood episodes in their lithium-treated counterparts. In lithium-free depressed patients, more episodes were associated with lower T₄ levels; whereas in their lithium-treated counterparts, longer illness duration was associated with higher TSH levels and females with lower T₃ levels. These results suggest that bipolar patients with and without lithium exposure differ in prevalence and association of hypothyroidism and may have different response to thyroid hormone therapy.