Objective: We determined the effect of subclinical hyperthyroidism (defined as low circulating TSH with normal serum free T4) and subclinical hypothyroidism (raised serum TSH with normal free T4) on fasting levels of blood lipids.
Design: Prospective study of lipid concentrations in patients identified as having abnormal TSH.
Patients: Patients were identified in a population screening study of those over 60 years, with persistently low TSH with normal free T4 (n = 27) or high TSH but normal free T4 (n = 57). Patients were matched to controls with normal serum TSH by age, sex and body mass index.
Measurements: Serum TSH, free T4, free T3, total cholesterol, low density lipoprotein (LDL) cholesterol and high density lipoprotein (HDL) cholesterol.
Results: Serum free T4 measurements were significantly higher in those with subclinical hyperthyroidism than in their controls (P < 0.001) and lower in those with subclinical hypothyroidism than in matched controls (P < 0.001). Measurement of fasting lipids in patients and controls revealed a marked (12.2%) reduction in serum total cholesterol in subclinical hyperthyroidism (P < 0.01); no significant difference in fasting lipids between patients with subclinical hypothyroidism and controls was observed.
Conclusions: Differences in free T4 between those with low or high TSH and controls with normal TSH suggest that abnormalities of TSH directly reflect thyroid hormone excess and deficiency. A reduction in cholesterol in those with subclinical hyperthyroidism suggests a direct influence of thyroid hormone excess on lipid metabolism in these patients.