In vitro responses of the epidermis to triiodothyronine.

The effect of exogenous triiodothyronine on replication and anabolic activity in human epidermis has been studied in vitro. Measurements of epidermal replication, as estimated by a labeling index technique and by thymidine incorporation into skin sheets, were increased (p less than 0.005) from the control value by exogenous triiodothyronine at twice its physiological unbound concentration. Higher concentrations of triiodothyronine did not alter these elevated values significantly. Epidermal anabolism, measured by the rates of incorporation of tritiated proline and histidine, and by histidine uptake assessed autoradiographically, responded similarly with increased values (p less than 0.005) compared to controls in specimens incubated with twice physiological concentrations of the hormone. Further increases in the hormone concentration failed to change these measurements significantly. These observations indicate that the epidermis does respond to exogenous triiodothyronine in vitro, and support the concept that there are receptors for triiodothyronine associated with epidermal cells.