Is Cystatin C a Marker of Glomerular Filtration Rate in Thyroid Dysfunction

In the recent literature, cystatin C has been advocated as a new and more accurate estimate of glomerular filtration rate (GFR) (1). Cystatin C is a 13-kDa endogenous cysteine proteinase inhibitor produced by all nucleated cells at a constant rate and broken down completely in the renal tubuli (2). Cystatin C concentrations are independent of age and body weight, and there is no need for urine collection for clearance estimations. Furthermore, serum concentrations of cystatin C are not influenced by malignancy or inflammation. In contrast, the often-used serum creatinine concentration is supposedly influenced by dietary intake, renal tubular metabolism, age, and variations in muscle mass. There are also various analytical difficulties with the widely used Jaffe colorimetric assay for creatinine. A slight decrease in GFR has been found in patients with hypothyroidism, which improved significantly after treatment (3)(4)(5). We wondered whether cystatin C would also be a good marker of renal …