Thyroid disorders are known to influence lipoprotein metabolism. In the current study we examined the incidence of thyroid function abnormalities in patients attending our outpatient lipid clinic. During the last 2 years, 248 patients were admitted to our lipid clinic for the diagnosis and management of dyslipidemia. In all cases, a detailed medical history was obtained and a thorough physical examination was performed with emphasis on the presence of symptoms/signs indicative of underlying thyroid diseases. In addition to lipid parameters, thyrotropin (TSH) and free thyroxine (FT4) levels were measured in a fasting blood sample. Seven female asymptomatic patients (2.8%) had frank biochemical hypothyroidism, and 11 patients (9 female, 2 male) (4.4%) had subclinical hypothyroidism with TSH levels between 5.8-19 mU/L. After restoration of a euthyroid state with levothyroxine therapy, no significant changes in serum lipid parameters were observed in the whole group of patients with subclinical hypothyroidism. However, in 4 patients with TSH levels >12 mU/L relatively small doses of levothyroxine (75 microg/day) were followed by a significant improvement of serum lipid profile. Interestingly, 3 patients exhibited clinical or subclinical hyperthyroidism that influenced serum lipid parameters as well as the effectiveness of hypolipidemic treatment. It is concluded that thyroid function abnormalities are relatively common in dyslipidemic patients attending a lipid clinic and could significantly affect the patients' lipid profile as well as the patients' management.