Acute cold exposure, leptin, and somatostatin analog (octreotide) modulate thyroid 5'-deiodinase activity

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 [sup.125]I from [sup.125]I-reverse triiodothyronine ([rT.sub.3]) under different assay conditions. Rats exposed to 4[degrees]C (15, 30, 60, and 120 rain) showed progressive reduction in thyroidal D1 and D2, reaching ~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 [micro]g/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 [micro]g/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. hypothyroidism; liver 5'-deiodinase; thyroid hormone