CHANGES IN SERUM THYROXINE, TRIIODOTHYRONINE, AND THYROTROPIN INDUCED BY LITHIUM IN NORMAL SUBJECTS AND IN RATS

Serum thyroxine (T4), triiodothyronine (T3) and thyrotropin (TSH) were measured in 4 normal subjects before, during and 2 wk after administration of lithium carbonate (1200 mg/day) for 6 wk. All subjects developed thyroid enlargement associated with increase in TSH. Serum T3 and T4 did not change significantly. In 3 hypothyroid subjects maintained on thyroxine replacement, Li administration did not alter circulating levels of T4 or T3, suggesting no effect of Li on the extrathyroidal metabolism of the thyroid hormones. In rats fed a low I diet containing 122 mg/kg per day lithium carbonate, goiter, increased 131I uptake, and a decrease in intrathyroidal T3 and T4 with an increase in MIT [monoiodotyrosine], DIT [diiodotyrosine] and MIT/DIT were observed at 3 wk. Serum T3 fell significantly only during the 3rd wk, while T4 did not change. Thyroid enlargement, secondary to a rise in TSH, appears to play a role in the adaptation of the thyroid gland to lithium carbonate. The exact mechanism for the elevation of TSH has not been elucidated.