title: Influence of thyroid function on different kidney function tests.
abstract: OBJECTIVE
  The commonly used kidney function tests have limitations, especially in thyroid dysfunction. Therefore, we studied the most commonly used kidney function tests in patients with hypo- and hyperthyroidism and after reaching euthyroidism.
  
  
  METHODS
  Prospective case series in 16 patients with thyroid dysfunction. Serum creatinine, 24-hour creatinine clearance, calculated glomerular filtration rate (GFR) by Cockroft-Gault, estimated GFR (eGFR) by the Chronic Kidney Disease Epidemiology Collaboration equation, serum cystatin C, eGFR based on cystatin C, eGFR based on a combined (cystatin C and creatinine) formula and plasma neutrophil gelatinase-associated lipocalin (NGAL) were measured in hypo- and hyperthyroidism and after gaining euthyroidism.
  
  
  RESULTS
  When free thyroxine (fT(4)) normalized in hypothyroid patients, creatinine decreased and creatinine-based eGFR increased significantly. In contrast, cystatin C increased and eGFR based on cystatin C decreased significantly. There was no significant change in NGAL levels. When fT(4) normalized in patients with hyperthyroidism, creatinine increased and creatinine-based eGFR decreased significantly. In contrast, cystatin C decreased and cystatin-C-based GFR increased significantly. There was no significant change in NGAL levels.
  
  
  CONCLUSIONS
  Thyroid function has a major influence on the vast majority of kidney function tests. Cystatin C is strongly influenced by the thyroid function and should be avoided in thyroid disorders. There was no effect on the plasma NGAL levels. The recommended kidney function test is a measurement of creatinine-based eGFR.
authors.affiliation: Division of General Internal Medicine and Nephrology, Department of Internal Medicine, Robert Bosch Hospital, Stuttgart, Germany. martin.kimmel@rbk.de
authors.forename: Martin
authors.initials: M
authors.lastname: Kimmel
authors.forename: Niko
authors.initials: N
authors.lastname: Braun
authors.forename: Mark Dominik
authors.initials: MD
authors.lastname: Alscher
mesh_terms: D000328:Adult; D000368:Aged; D005260:Female; D006801:Humans; D006980:Hyperthyroidism / Q000097:blood / Q000175:diagnosis; D007037:Hypothyroidism / Q000097:blood / Q000175:diagnosis; D007677:Kidney Function Tests / Q000379:methods*; D008297:Male; D008875:Middle Aged; D011446:Prospective Studies; D013960:Thyroid Function Tests / Q000379:methods*; D013961:Thyroid Gland / Q000502:physiology*; D055815:Young Adult