Thyroid hormone has multiple effects on the regulation of lipid synthesis, absorption, and metabolism. Studies consistently demonstrate elevated levels of serum total cholesterol, low-density lipoprotein cholesterol (LDL-C), apolipoprotein B, lipoprotein(a), and possibly triglycerides in individuals with overt hypothyroidism, all of which are reversible with levothyroxine therapy. Although it is estimated that 1 to 11% of all patients with dyslipidemia have subclinical hypothyroidism, the effects of subclinical hypothyroidism on serum lipid values are less clear. Apolipoprotein B levels may be increased in patients with subclinical hypothyroidism. Although some studies have demonstrated that total cholesterol and LDL-C levels are elevated in patients with subclinical hypothyroidism, others have not shown any effect of subclinical hypothyroidism on these lipid measurements. Serum triglycerides, lipid subparticle size, and LDL-C oxidizability may be altered in subclinical hypothyroidism, but these studies have also been inconsistent. The preponderance of evidence suggests that HDL-C and lipoprotein(a) levels are not altered in subclinically hypothyroid patients. Smoking and insulin resistance may modify the effects of subclinical hypothyroidism on serum lipid values. Clinical trials to date have not consistently shown a beneficial effect of levothyroxine treatment on serum lipid levels in subclinically hypothyroid patients.