Reduced cystatin C-estimated GFR and increased creatinine-estimated GFR in comparison with iohexol-estimated GFR in a hyperthyroid patient: A case report

INTRODUCTION Estimation of the glomerular filtration rate (GFR) is essential for the evaluation of patients with kidney disease, and for treating patients with drugs that are eliminated from the circulation by the kidneys. Cystatin C has been shown to be superior to creatinine for estimating GFR in several studies. However, studies showing that thyroid function has an impact on cystatin C have not addressed the question of whether the changes in cystatin C levels are due to changes in GFR or in cystatin C synthesis. CASE PRESENTATION We report an account of a hyperthyroid patient with a discrepancy between the GFR estimates from cystatin C and creatinine. The cystatin C concentration (1.36 mg/L) was higher and gave an estimated GFR which was lower (51 mL/min/1.73 m2), while the creatinine concentration was lower (36 mumol/L) and gave a corresponding creatinine-estimated GFR that was higher (145 mL/min/1.73 m2) than the iohexol-estimated GFR (121 mL/min/1.73 m2) during the hyperthyroid period. After thyroidectomy, the creatinine concentration was 36 mumol/L and creatinine-estimated GFR was calculated as 73 mL/min/1.73 m2, while the cystatin C concentration and cystatin C-calculated GFR was 0.78 mg/L and 114 mL/min/1.73 m2, respectively. CONCLUSION In contrast to creatinine, cystatin C levels rose in the hyperthyroid state as compared to the euthyroid state. The cystatin C-estimated GFR was reduced compared to the iohexol-estimated GFR. This patient case shows that the hyperthyroid-associated changes in cystatin C levels are not due to changes in GFR. Thyroid function should thus be considered when both cystatin C and creatinine are used as markers of kidney function.