Association of longitudinal changes in serum lipids with the natural history of subclinical hypothyroidism: A retrospective cohort study using data from the REACTION study.

Background: Subclinical hypothyroidism (SCH) often leads to alterations in lipid profile, which may negatively impact humans health. Whether lipids in turn affect the natural history of SCH is unknown. We aimed to assess the association between longitudinal changes in serum lipid levels and the natural history of SCH. Methods: This retrospective cohort study using data from the REACTION study included 581 patients with SCH who were enrolled between July 1, 2011, and December 19, 2014, with a median follow-up of three [IQR, 2·86-3·21] years. Patients with missing data or conditions that can affect thyroid function were excluded. Changes in serum lipid levels were calculated from serum lipid measurements 3 years apart and classified in two ways: 1) the first, second, and third tertiles of the difference between baseline and follow-up and 2) the percent change from baseline, namely, serum lipid decrease ≥ 25%, minor change, and serum lipid increase ≥ 25%. The natural history of SCH includes regression to euthyroidism, SCH persistence, or progression to overt hypothyroidism (OH). Odds ratios (ORs) were estimated by multivariable logistic regression. Validation was performed on data from a health management cohort study conducted from January 1, 2012, to December 31, 2016, with a median follow-up of two [IQR, 1·92-2·08] years. After using the same inclusion and exclusion criteria as the REACTION cohort study, 412 patients with SCH were eligible for the validation analysis. Findings: There were 132 (22·7%) men and 449 (77·3%) women in the study, with a median age of 56 [IQR,49-62] years. During follow-up, 270 (46·5%), 266 (45·8%), and 27 (4·6%) patients had regression to euthyroidism, persistent SCH, and progression to OH, respectively. Both grouping manners showed a significant association between changes in lipid levels and the natural history of SCH. A total cholesterol (TC)-level increase was independently associated with a greater risk of progression to OH (OR for ≥ 25% TC increase vs. minor change: 5·40; 95% CI 1·46-21·65), whereas TC-level declines increased the likelihood of regressing to euthyroidism (OR for ≥ 25% TC decrease vs. minor change: 3·45; 95% CI 1·09-12·43). Similarly, the likelihood of regression according to changes in triglyceride (TG) levels exhibited a consistent trend with that according to TC-level changes. A similar pattern of association was observed in the validation cohort. Interpretation: Changes in serum lipid levels in SCH are associated with future progression or regression risk, suggesting that the changes in serum lipid levels may affect the natural history of SCH. Clinicians should pay attention to the long-term control of serum lipids levels in populations with SCH, which may benefit thyroid function. Funding: This work was supported by grants from the National Key Research and Development Program of China (2017YFC1309800), the National Natural Science Foundation (81430020, 82070818), and the "Outstanding University Driven by Talents" Program and Academic Promotion Program of Shandong First Medical University (2019LJ007).