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 the histology of skin in hypothyroid states is well documented, the literature contains little assessment of skin in thyrotoxic states. In light of the paucity of information on skin under the influence of excess thyroid hormone, we investigated the direct effect of thyroid hormone on skin. Triiodothyronine (T₃) was applied topically daily in liposomes to SKH-1 hairless mice for 7 days and to CD rats for 2 weeks. There was a dose-dependent increase in epidermal proliferation, dermal thickening, and hair growth in T₃-treated animals. Mice that received 3.8 μg of T₃ had 42% more hairs per millimeter than controls (p < 0.01), hair length that was 1,180% longer (p < 0.001), 49% greater epidermal ³H-thymidine incorporation (p < 0.01), and 80% more 5-bromo-2′-deoxyuridine (BrdU) stained cells (p < 0.05). Rats receiving 12.8 μg T₃ had 48% greater dermal thickness than controls (p < 0.001), 26% greater epidermal thickness (p < 0.001), 85% more hairs per millimeter (p < 0.005), and 130% greater ³H-thymidine incorporation into the epidermis (p < 0.01). Thus, topically applied thyroid hormone has dramatic effects on both skin and hair growth. These observations offer a new strategy for developing thyroid hormone and its analogues for treating disorders of skin and hair growth.