Paradoxical Changes in Cystatin C and Serum Creatinine in Patients with Hypo- and Hyperthyroidism

In the ongoing search for improved serum markers of impaired renal function (1), the low-molecular-weight protein cystatin C has been advocated as a promising and probably superior alternative to creatinine when used to assess glomerular filtration rate (GFR) (2)(3). Cystatin C possesses most of the properties of an ideal GFR test in that it is produced by all nucleated cells at an apparently constant rate, is freely filtered at the glomerulus, and is then fully destroyed in the proximal renal tubule (4). Its rate of production is not influenced by inflammation or malignancy and, unlike creatinine, is unaffected by the muscle mass, sex, or age of a patient (5). Indeed, studies to date would seem to confirm the potential of the marker in many clinical situations in which an accurate estimate of GFR is required in both adults (5)(6)(7) and children (8)(9). Before cystatin C measurement became widespread, another low-molecular-weight protein, β2-microglobulin, was promoted as a marker of GFR for similar reasons (10)(11)(12), but in contrast to cystatin C, its usefulness was subsequently found to be limited by the increased serum values found in inflammatory and neoplastic conditions (13). Thyroid disease …