Objective: To determine whether the literature supports an effect of subclinical hypothyroidism on serum lipids and, if so, what are the effects of thyroxine replacement therapy.
Data sources: Articles were identified on MEDLINE using the MeSH terms hypothyroidism, lipids, or cholesterol.
Data synthesis: The majority of studies that determined the prevalence of lipid abnormalities in subclinical hypothyroidism and studies that evaluated the effects of thyroxine replacement on lipids were small, uncontrolled, and varied in inclusion criteria. Six randomized, placebo-controlled trials were identified that evaluated the effect of levothyroxine on lipids in subclinically hypothyroid patients.
Conclusions: Subclinical hypothyroidism can potentially contribute to a pro-atherogenic lipid profile, with effects being greater at higher thyroid-stimulating hormone levels. Thyroxine replacement reduces total cholesterol and low-density lipoprotein cholesterol, with no effect on triglycerides. Effects on high-density lipoprotein, lipoprotein (a), and apolipoproteins A1 and B require further study. Larger prospective studies are needed to clarify many issues.