INTRODUCTION Thyroid peroxidase activity inhibiting immunoglobulins (anti-TPO Ab) is a sign of autoimmune process in the thyroid gland. Association of hyperthyroidism and diabetes mellitus has been classically described. However, hypometabolic state, as a consequence of hypothyroidism, is not frequently linked with the biological activity of insulin. CASE DESCRIPTION A 51-year old man was admitted to the Clinic with unregulated diabetes, untreated for 5 yrs. Insulin therapy was introduced one year before, with 96 units on admission. He had bowel movements every three days. BH 176cm, BW 120kg, a puffy face and swollen body. Fundus examination did not show specific diabetic leasions. Hepatic steatosis was present on ultrasound examination. Occlusion of coronary arteries and superficial femoral artreries was present on angiography, and stenosis of carotid artreies on doppler duplex examination. HbA1c 14.7%. TSH 85.7 mlU/l, FT4 1.6 pmol/l, FT3 1.4. Anti TPO Ab >600 IU/ml, triglycerides 2.26 mmol/l, HDL 1.15, cholesterolemia 10.0. Levothyroxine substitution was introduced starting with 25 mgr, gradually increasing up to 75 mgr. The need for insulin gradually decreased and finally it was switched to glibenclamide 5mg +0+2.5 mg. On discharge his FBG was 7.0 mmol/l. HOMA -B 52.3, HOMA-R 9.8. DISCUSSION We can conclude that in our patient secondary obesity caused deterioration of diabetes. After introduction of substitution therapy with levothyroxine, decrease of insulin resistance and of cholesterol level was established. The duration of undiagnosed hypothyroidism can be a matter of speculation. However, the beneficial effect of normalized metabolism on atherosclerotic process will be obvious in the future.