Many studies have found clinical and metabolic alterations in subclinical hypothyroidism, however, there are disagreements about the benefits of levothyroxine therapy. The objective of the present study was to analyze the effects of 6 months of treatment on the lipid profile of patients with subclinical hypothyroidism. A randomized double blind, placebo-controlled clinical assay was conducted. Sixty patients were enrolled in stratified random allocation by TSH levels that generated similar groups in average: free thyroxine levels, lipid levels, age, clinical score, and sedentary. At 6 months, 18 patients in the levothyroxine and 20 in the placebo group were reevaluated and a fall in all atherogenic lipid variables was observed with treatment. The TC and LDL-c variations (-22.6+/-37.2 and -18.5+/-34.6 mg/dl, respectively) in the group that received LT4 were statistically different (p=0.023 and p=0.012) from those occurring in the placebo group (+7.3+/-37.1 and +14.7+/-40.6 mg/dl). Baseline characteristics associated with better improvement in the levels of TC and LDL-c were the presence of TPO-Ab, TSH levels >8.0 microUI/ml, Body Mass Index >or=25 kg/m2, and the presence of menopause. We concluded that treatment with dose-adjusted levothyroxine reduced atherogenic lipid levels in some patients. Further studies to determine the effects of LT4 replacement in specific subgroups of SH patients are still necessary, especially in patients with TSH <8.0 microUI/ml.