The primary mechanism for hypercholesterolemia in hypothyroidism is accumulation of LDL cholesterol due to a reduction in the number of cell surface receptors for LDL, resulting in decreased catabolism of LDL. A decrease in LDL receptor activity has also been described. A different mechanism, reduced lipoprotein lipase activity, is responsible for development of hypertriglyceridemia in hypothyroidism.
According to the study at Boston University Medical Center: "Hypothyroidism and dyslipidemia: modern concepts and approaches" [PubMed Abstract, 2004 Nov;6(6):451-6]: "The effects of subclinical hypothyroidism on serum lipid values are less clear. The preponderance of evidence suggests that total cholesterol, LDL cholesterol, and possibly triglycerides are increased in patients with subclinical hypothyroidism, whereas high-density-lipoprotein (HDL - "the good" - cholesterol) cholesterol and lipoprotein(a) remain unchanged".[email protected]