The significance of subclinical hypothyroidism in regard to ensuing hyperlipidemia remains unclear. Because an unfavorable lipid profile would provide a possible explanation for the reported association of coronary-heart disease with this syndrome, we have evaluated the relationship of thyrotropin (TSH) with total cholesterol, low-density-lipoprotein (LDL) cholesterol, and triglycerides in patients with normal thyroid function (n = 4886) as well as subclinical (n = 1055) and manifest (n = 92) hypothyroidism. Serum concentrations of LDL cholesterol were similar in euthyroid persons (134+/-39 mg/dL) and in patients with subclinical hypothyroidism (137+/-40 mg/dL) but were higher (178+/-70 mg/dL, p < 0.01) in overt hypothyroidism. Within the group of subjects with subclinical hypothyroidism there was no apparent relationship between serum concentrations of TSH ranging from 4.0 to 49.0 microU/mL and concentrations of LDL cholesterol. Thus, there is no "threshold value" of TSH in these patients per se necessitating substitution therapy with thyroxine.