Serum lipid levels in relation to serum thyroid-stimulating hormone and the effect of thyroxine treatment on serum lipid levels in subjects with subclinical hypothyroidism: the Tromso Study

Objective. To evaluate the relation between serum thyroid-stimulating hormone (TSH) and lipids. Design. Cross-sectional epidemiological study, nested case-control study, and a placebo-controlled double-blind intervention study. Methods. In the 5th Tromso study serum TSH, total cholesterol (TC), triglycerides (TG), high-density lipoprotein cholesterol (HDL-C), and low-density lipoprotein cholesterol (LDL-C) were measured. Subjects with subclinical hypothyroidism (SHT) and a matching control group were re-examined and apolipoprotein A1 (Apo A1) and apolipoprotein B (Apo B) were also measured. Subjects with SHT were included in an intervention study with thyroxine supplementation for 1 year. Results. A total of 5143 subjects from the 5th Tromso study were included. A significant and positive correlation between serum TSH levels and serum TC and LDL-C levels were found in both genders. However, in the females this did not reach statistical significance after adjusting for age and BMI. The serum LDL-C were significantly higher and the Apo A1 levels significantly lower in 84 SHT subjects compared with 145 controls, and in the SHT females the TC levels were also significantly elevated. In the intervention study (32 subjects given thyroxine and 32 subjects given placebo), we observed a significant reduction in the Apo B levels after thyroxine medication. In those that at the end of the study had serum TSH levels in the range 0.2-2.0 mIU L-1, the serum TC and LDL-C levels were also significantly reduced. Conclusions. There is a positive association between serum TSH levels and TC and LDL-C levels. These lipid levels are reduced with thyroxine treatment in subjects with SHT.