Hyperhomocysteinemia is a risk factor for premature atherosclerotic vascular diseases. It is known that plasma homocysteine levels are higher in hypothyroid patients compared to healthy subjects. The aim of our study was to assess plasma total homocysteine concentrations in hyperthyroid patients before and after treatment when euthyroid status was reached and compare them with control group. Thirteen hyperthyroid patients (age, 42.9 +/- 15.6 year) and eleven healthy subjects (age, 39.9 +/- 12.5 year) were involved in the study. Plasma levels of homocysteine and serum cholesterol, triglyceride, HDL cholesterol, urea, creatinine, vitamin B12, folate were measured before and after treatment. LDL cholesterol and creatinine clearances were calculated. Pretreatment homocycteine levels of the hyperthyroid patients were significantly lower than healthy controls (11.5 +/- 3.6 micromol/L vs. 15.1 +/- 4.5 micromol/L, respectively, p<0.05). Posttreatment homocysteine levels were significantly higher than pretreatment levels (13.9 +/- 6.3 micromol/L vs. 11.5 +/- 3.6 micromol/L, respectively, p<0.05) and posttreatment creatinine clearance were lower than pretreatment level (103.5 +/- 12.7 ml/min vs. 114.2 +/- 9.3 ml/min, respectively, p<0.01). Lower homocysteine levels in hyperthyroidism can be partially explained with the changes in creatinine clearance.