A Cross-sectional Study of the Association Between Circulating TSH Level and Lipid Profile in a Large Spanish Population

Objective: Some evidence suggests that high serum TSH levels are associated with an adverse lipid profile, but this association is not clear when plasma TSH is within the reference range. Nevertheless, these studies have never been conducted in Spain, a country with a strong adherence to the Mediterranean diet. The study aim was to analyse the association between blood TSH levels and circulating lipids in a large Spanish population and set up a TSH reference range in different age, gender and Body Mass Index (BMI) subpopulations from our cohort.

Design: Cross-sectional study on 20 783 subjects.

Patients: We analysed circulating levels of total cholesterol (TC), low-density lipoprotein cholesterol (LDLc), high-density lipoprotein cholesterol (HDLc) and triglycerides (TG) and compared them with TSH serum levels. Discriminant function analysis was used to determine the TSH cut-off level from where hyperlipidaemia developed.

Results: In individuals free of thyroid dysfunction, the mean and the 95% TSH (mU/l) reference limits were 2·20 and 0·72-4·43, respectively. We observed a sex-related difference in TSH concentration (men, 2·07 and 0·72-4·29; women, 2·29 and 0·72-4·49; P < 0·01). We also observed a weight-related difference in TSH concentration (BMI < 30 kg/m(2) , 2·16 and 0·72-4·39; BMI ≥ 30 kg/m(2) , 2·28 and 0·71-4·47; P < 0·01). TSH was positively associated with TC, TG and LDLc levels and negatively with HDLc.

Conclusion: We found an association between TSH and lipids in that as TSH increased, the lipid profile became less favourable, even within the normal range. Additionally, TSH reference ranges varied according to gender, age and BMI.