ESTIMATION OF GLOMERULAR FILTRATION RATE FROM SERUM CYSTATIN C AND CREATININE IN PATIENTS WITH THYROID DYSFUNCTION

Given that thyroid function influences serum cystatin C and creatinine levels, the question arises as to whether it is possible to accurately estimate glomerular filtration rate (GFR) in patients with thyroid dysfunction. The objective of the study was to determine serum cystatin C and creatinine levels and estimate GFR in patients with thyroid dysfunction. The study included 32 cases with newly diagnosed hyperthyroidism and 27 cases with newly diagnosed hypothyroidism, as well as 20 healthy controls matched for sex and age with the cases. Serum concentrations of thyroid stimulating hormone (TSH), free triiodothyronine (fT3) and free thyroxine (fT4), creatinine and cystatin C were measured in all study subjects. GFR was estimated using the Modification of Diet in Renal Disease (MDRD), the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) and cystatin C-based equations. Serum cystatin C levels were significantly higher in hyperthyroid subjects compared to controls (1.32 +/- 0.31 vs. 0.89 +/- 0.15; p<0.01). Serum creatinine levels were significantly lower in hyperthyroid subjects compared to controls (60.6 +/- 10.2 vs. 76.4 +/- 8.6; p<0.01), and significantly higher in hypothyroid subjects compared to controls (94.5 +/- 13.2 vs. 76.4 +/- 8.6; p<0.01). GFR estimated with the MDRD equations was significantly higher in hyperthyroid subjects compared to hypothyroid subjects (101.6 +/- 20.7 vs. 64.1 +/- 11.6 mL/min/1.73m(2); p<0.01). GFR estimated with the equation based on serum cystatin C was significantly lower in hyperthyroid subjects compared to hypothyroid subjects (59.2 +/- 22.1 vs. 92.1 +/- 16.0 mL/min/1.73m(2); p<0.01). Although serum cystatin C is regarded a reliable marker of GFR and more sensitive than serum creatinine, it has limitations in patients with thyroid dysfunction, due to significant changes in its serum concentrations regardless of renal function. In patients with thyroid dysfunction GFR should therefore be estimated using the equations based on serum creatinine.