BACKGROUND: Hypothyroidism, both overt and subclinical, is associated with dyslipidemia and increased cardiovascular risk which is not completely corrected by thyroid hormone replacement therapy. In patients with subclinical hypothyroidism, when TSH levels are <10 mIU/L, dyslipidemia is considered a relative indication for establishment of replacement therapy. Although reasonable, the evidence supporting this approach is limited, and the results are conflicting. Here we present the results of a retrospective study directed to assess the differences in lipid metabolism parameters between patients with subclinical hypothyroidism treated or untreated with levothyroxine. METHODS: Single-center, retrospective cohort study. Patients with diagnosis of mild subclinical hypothyroidism not taking thyroid hormone replacement therapy (at baseline) and at least two measurements of TSH and lipid panel were included in the analysis. Patients taking lipid lowering therapies were excluded from the study. Primary endpoints were changes in TSH, total and LDL cholesterol between patients treated and untreated with thyroid hormone replacement therapy. RESULTS: Fifty-two patients (14 treated and 38 untreated) median age 61.0 years and body mass index 27.0 kg/m2 were included in the analysis. Median baseline TSH and LDL-cholesterol were 5.6 mIU/L and 121.0 mg/dL, respectively. At follow up, median TSH decreased in both groups (treated -2.5 mIU/L [95% CI: -3.8, -1.2] untreated -1.7 mIU/L [-2.6, -0.8] p<0.001). Median total and LDL-cholesterol significantly increased in the treated group (7.8 mg/dL [2.2, 13.5] p=0.01 and 13.2 mg/dL [1.4, 25.1] p=0.03, respectively), while the untreated group saw no changes. No changes were observed in HDL-cholesterol and triglycerides for either group. CONCLUSIONS: Our results indicate that in patients affected by mild subclinical hypothyroidism not receiving lipid lowering treatments, levothyroxine therapy was associated with increase in total cholesterol and LDL-cholesterol, while this did not occur in untreated patients. One possible explanation of this counterintuitive finding is the inhibition of endogenous thyroid hormone production with further reduction of T3-mediated metabolic on lipid metabolism. Prospective interventional studies are needed to confirm this observation, potentially changing the approach to the treatment of subclinical hypothyroidism.