SUN-400 Treatment of Subclinical Hypothyroidism and its Impact on Lipid Metabolism

Abstract Disclosure: R.A. Zielinski: None. M. Dillon: None. M. Katsetos: None. S. Sacco: None. F.S. Celi: None. Introduction: There are no clear consensus guidelines for treating subclinical hypothyroidism (SH) when the TSH is <10 mIU/L. Hypothyroidism, both overt and subclinical, is associated with hypercholesterolemia. While dyslipidemia is considered by some an indication for initiating therapy in SH, some studies suggest that this is not effective. Hypothesis: We hypothesized that patients with SH who are treated with levothyroxine (LT4) will develop a paradoxical increase in cholesterol due to decreased endogenous T3 production. Methods: In this single-center retrospective study, we compared patients treated and not treated with LT4. We reviewed our outpatient records between 2021-2024 for patients with an SH diagnosis and at least two outpatient encounters with records of TSH and lipid panel analyses. Exclusion criteria were current chemotherapy, use of statins, use of desiccated thyroid extracts or liothyronine, and the absence of an initial TSH value between 4.5 mIU/L and 10 mIU/L. We next evaluated changes in TSH, total cholesterol, LDL, HDL, and triglyceride values between groups at baseline and follow-up (next) lipid panels. Results: Our final cohort included 26 treated and 41 untreated patients. Most patients were White or Caucasian females, with a median age of 59 and BMI of 26 kg/m2. Median baseline TSH and LDL-cholesterol were 5.5 mIU/L and 122 mg/dL, respectively. Treated patients were younger (median age=47; untreated=63; p=0.02). The median duration between baseline and follow-up was 18 months. TSH decreased in both groups (p<0.03), and did not differ (p=0.67) between groups. Untreated patients had decreased total cholesterol at follow-up (M=-9.9 [95% CI: -19.2, -0.6], p=0.04), which was significantly lower than treated patients, who saw no significant change (p=0.05). Treated patients had increased LDL at follow-up (M=12.6 [95% CI: 1.2, 23.9], p=0.04) and nontreated patients had decreased LDL (M=-14.2 [95% CI: -22.2, -6.2], p=0.001), which significantly differed (p=0.003) between groups. HDL and triglycerides did not change over time for either group (p>0.11). Conclusion: Our study found that LDL increased in LT4-treated patients and decreased in untreated patients. While our study is limited by the small sample size, it does beg the question of whether LT4 treatment for SH is beneficial, and if it could paradoxically increase cholesterol levels, and potentially cardiovascular risk. Presentation: Sunday, July 13, 2025