Topical triiodothyronine stimulates epidermal proliferation, dermal thickening, and hair growth in mice and rats

The skin is a classic target tissue for thyroid hormone action. Although th e histology of skin in hypothyroid states is well documented, the literatur e contains little assessment of skin in thyrotoxic states. In light of the paucity of information on skin under the influence of excess thyroid hormon e, we investigated the direct effect of thyroid hormone on skin. Triiodothy ronine (T-3) was applied topically daily in liposomes to SKH-1 hairless mic e for 7 days and to CD rats for 2 weeks. There was a dose-dependent increas e in epidermal proliferation, dermal thickening, and hair growth in T-3-tre ated animals. Mice that received 3.8 mug of T-3 had 42% more hairs per mill imeter than controls (p < 0.01), hair length that was 1,180% longer (p < 0. 001), 49% greater epidermal H-3-thymidine incorporation (p < 0.01), and 80% more 5-bromo-2<prime>-deoxyuridine (BrdU) stained cells (p < 0.05). Rats r eceiving 12.8 <mu>g T-3 had 48% greater dermal thickness than controls (p < 0.001), 26% greater epidermal thickness (p < 0.001), 85% more hairs per mi llimeter (p < 0.005), and 130%. greater H-3-thymidine incorporation into th e epidermis (p < 0.01). Thus, topically applied thyroid hormone has dramati c effects on both skin and hair growth. These observations offer a new stra tegy for developing thyroid hormone and its analogues for treating disorder s of skin and hair growth.