[Thyroid disorders and dyslipidemias]

While overt thyroid disturbances, characterized by symptoms and/or clinical signs with abnormal serum levels of thyroid hormones, are generally associated with perturbations in the lipid profile, the situation is less clear as far as subclinical thyroid disturbances, defined by isolated abnormalities of thyroid stimulating hormone (TSH) levels, are concerned. In severe hyperthyroidism, a decrease of total cholesterol, LDL cholesterol and apoprotein B concentrations is generally observed. These biological parameters are normalized when appropriate antithyroid treatment is given. In profound hypothyroidism, on the contrary, elevated levels of total and LDL cholesterol levels are observed, which decrease after hormonal replacement. In both cases, the changes in serum levels of HDL cholesterol, triglycerides and lipoprotein (a) are less systematic, both before and after treatment. Lipid abnormalities associated with subclinical thyroid disturbances remain controversial. However, two recent meta-analyses have shown higher LDL cholesterol levels in presence of subclinical hypothyroidism and a significant reduction of such lipid abnormality after administration of thyroxine. Furthermore, they demonstrated a higher prevalence of subclinical hypothyroidism in a population with hypercholesterolaemia when compared to a population with normal cholesterol levels. Finally, a significant reduction in both total and LDL cholesterol concentrations has been reported after administration of thyroxine in a small group of hypercholesterolaemic patients with basal TSH levels in the upper range of normal values. In view of the results of the literature, strategies are proposed to help the clinician in the management of patients with overt or subclinical thyroid disturbances, associated with dyslipidaemia.