Impaired thyroid function-hypothyroidism, is associated with changes in the spectrum of serum lipids. Thyroid hormones increase the number and activity of LDL receptors in the liver and peripheral tissues and have also a direct impact on the triacylglycerol synthesis in the liver. If their amount in the organism is inadequate, the metabolic turnover of LDL-cholesterol and triacylglycerols is reduced and subsequently a rise of the LDL-cholesterol and triacylglycerol plasma concentration occurs. The patients are then frequently referred with the diagnosis of hyperlipoproteinaemia to a lipid clinic for treatment, the hypothyroidism remaining undetected. In the submitted study the authors had the objective to assess how many patients referred to lipid clinics on account of a high cholesterol level suffered from undetected and thus untreated hypothyroidism. Ex post it was decided to examine the thyroid function in all patients attending the lipid clinic, IPZV Zlín, incl. those who are on the records and had treatment for several years. It proved thus possible to assemble a relatively large group of patients.