- Transdermal delivery of insulin is a non-invasive alternative to the subcutaneous injection of insulin in diabetic patients. It has been found that skin pretreatment with iodine followed by a dermal application of insulin results in reduced glucose and elevated hormone levels in the plasma. Topical iodine protects the dermally applied insulin presumably by inactivation of endogenous sulfhydryls such as glutathione and gamma glutamylcysteine which can reduce the disulfide bonds of the hormone. Thus, the effect of iodine is mediated by retaining the potency of the hormone during its penetration via the skin into the circulation. The proposed procedure might be applicable for additional disulfide-containing peptides such as calcitonine, somatostatin, oxytocin/vasopressin and their analogs.