It is recognised that leptin is a component of a series of important thyroid cell processes, including the regulation of TSH expression [31] and the development of autoimmunity [32, 33], and that serum leptin levels may be related to the presence of insulin resistance in hypothyroidism [34] and to other diseases that may be associated with thyroid disorders, such as obesity and insulin resistance. The gene discussed is LEP; the disease is obesity due to melanocortin 4 receptor deficiency.