[Thyroid hormones and the metabolism of lipoproteins]

Variations, generally not very marked, observed in triglycerides levels during thyroid affections are the result of the action of thyroid hormones on very low density lipoprotein (VLDL) metabolism. Hepatic synthesis of VLDL is increased in hyperparathyroidism due to elevation of circulating fatty acids levels, and in hypothyroidism only in obese patients. Catabolism of VLDL is accelerated in hyperthyroidism and slowed in hypothyroidism. The variations are probably related to changes in activity of lipoprotein lipase and/or hepatic triglyceride lipase, as well as structural modifications in VLDL. Variations in cholesterolemia observed during thyroid affections result mainly from alterations in low density lipoprotein (LDL) metabolism, parallel variations occurring in apoprotein B levels. Thyroid hormones increase cholesterol synthesis, and accelerate LDL catabolism by increasing the number of apoprotein B receptor in peripheral tissues and probably by other mechanisms independent of these receptors. Significant variations in cholesterol levels carried by high density lipoproteins (HDL) are not observed during hypothyroidism, an inconstant reduction being noted in hyperthyroidism. Apoprotein A2 levels are unchanged in thyroid disorders, those of apoproteins A1 being reduced in hyperthyroidism and normal in hypothyroidism, though they diminish transiently during substitutive treatment of the latter condition.