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 (1,200 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 maintained on thyroxine replacement, lithium administration did not alter circulating levels of T4 or T3, suggesting no effect of lithium on the extrathyroidal metabolism of the thyroid hormones. In rats fed a low iodine diet containing 122 mg/kg/day lithium carbonate, goiter, increased 131I uptake, and a decrease in intrathyroidal T3 and T4 with an increase in MIT, DIT, and MIT/DIT were observed at 3 wk. Serum T3 fell significantly only during the third week, 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.