We investigated the effect of acute cold exposure, leptin, and the somatostatin analog octreotide (OCT) on thyroid type I (D1) and II (D2) deiodinase activities. Microsomal D1 and D2 activities were measured by the release of (125)I from (125)I-reverse triiodothyronine (rT(3)) under different assay conditions. Rats exposed to 4 degrees C (15, 30, 60, and 120 min) showed progressive reduction in thyroidal D1 and D2, reaching approximately 40% at 2 h (P < 0.05) despite increased circulating TSH (P < 0,05) associated with the higher thyroid D1 and D2 in hypothyroid rats. A single injection of leptin (8 microg/100 g body wt sc) induced increased thyroid and liver D1 (P < 0.05), but not thyroid D2, activities at 30 and 120 min, independently of the serum TSH rise shown only at 2 h. OCT (1 microg/kg body wt sc) increased D1 and D2 activity significantly 24 h after a single injection, with no changes in serum TSH. Therefore, leptin and somatostatin are potential physiological upregulators of thyroid deiodinases, and their low secretion during acute cold exposure may be a potential mechanism contributing to cold-induced reduction in thyroid deiodinase activity.