Objective   To evaluate the activity of antioxidant paraoxonase 1 (PON1) and the oxidative stress status in patients with subclinical hypothyroidism (SCH)

and the relationship between PON1 activity and oxidative stress parameters.   Methods   The fasting plasma levels of free triiodothyronine (FT

3

)

free thyroxine (FT

4

)

thyroid stimulating hormone (TSH)

PON1 activity

superoxide dismu

tase (SOD) and malondialdehyde (MDA)

oxidized low-density lipoprotein (ox-LDL) and lipids in 56 subclinical hypothyroidism patients and 48 matched normal controls were determined respectively. The correlation analysis was made between these variables.   Results   Compared with control

subclinical hypothyroidism had significantly lower PON1 [(142±56) kU/L vs (167±60) kU/L

]

(P0.01)

and SOD activity [(83.2±26.1) U/ml vs (93.1±28.2) U/ml

]

(P0.05)

and significantly higher MDA [(14.10±3.01)  μmol/L vs (10.25±3.34)  μmol/L

]

(P0.01)

and ox-LDL level [(596.2±68.1)  μg/L vs (467.2±60.1)  μg/L

]

(P0.01). Linear correlation analysis showed PON1 was positively correlated with SOD (r=0.302

P0.05)

and negatively correlated with ox-LDL (r=-0. 398

P0.01)

MDA (r=-0.401

P0.01) in subclinical hypothyroidism.   Conclusion   Plasma PON1 activity is significantly decreased in patients with subclinical hypothyroidism. Increased reactive oxygen species (ROS) levels in these patients may result in a pro-oxidation environment

which in turn leads to decrease PON1 activity. PON1 may participate in the pathophysiologic process of subclinical hypothyroidism through reducing antioxidant properties.