OBJECTIVE: The association between TSH and serum lipids in people with no apparent thyroid disease is insufficiently understood. We have studied the association between normal thyroid function, defined as TSH within the reference range of a general population, and concentrations of serum lipids. DESIGN: Cross-sectional, population-based study with 30,656 individuals without known thyroid disease. METHODS: Using general linear models, we calculated mean concentrations of total serum cholesterol, low-density lipoprotein (LDL) cholesterol, non-high-density lipoprotein (HDL) cholesterol, HDL cholesterol and triglycerides across categories of TSH. RESULTS: Within the reference range of TSH, there was a linear and significant (P for trend <0.001) increase in total serum cholesterol, LDL cholesterol, non-HDL cholesterol and triglycerides, and a linear decrease (P for trend <0.001) in HDL cholesterol with increasing TSH. Subgroup analyses showed statistically significant associations for all lipids in men above 50 years of age, and for triglycerides in all age groups. For women, associations were statistically significant in all age groups except for HDL cholesterol in women below 50 years of age. The associations with triglycerides and HDL cholesterol were stronger among overweight than normal weight individuals. CONCLUSIONS: Within the range of TSH that is considered clinically normal, we found that increasing level of TSH was associated with less favourable lipid concentrations. The association with serum lipids was linear across the entire reference range of TSH.