The syndrome of resistance to thyroid hormone (RTH) is characterized by elevated serum free thyroid hormones (FT4 and FT3) in the presence of unsuppressed TSH levels, reflecting resistance to the normal negative feedback mechanisms in the hypothalamus and pituitary. The degree of resistance within peripheral tissues determines whether thyrotoxic clinical features are associated with this condition. Classic features include attention deficit hyperactivity disorder, growth delay, tachycardia, and goiter. However, other features, such as frequent ear, nose and throat infections, hearing deficit, and decreased bone mass have recently been recognized. The phenotype of RTH is variable, with most patients presenting with mild to moderate symptoms. In this report we describe a girl with familiar RTH and diabetes mellitus. This is, to our knowledge, the first report regarding this association. Nearly one year after long-term triiodothyroacetic acid (Triac) therapy, we observed a reduction of thyroid hormone levels with an amelioration of insulin resistance. The possible interactions between these disorders are discussed.