Serum cystatin C is sensitive to small changes in thyroid function

Background: Serum cystatin C (CysC) is a novel marker for kidney function. The impact of mild thyroid dysfunction on CysC has never been investigated. Methods: CysC was determined at the time of diagnosis of subclinical hypo- and hyperthyroidism, and when TSH returned into the normal range in 40 consecutive patients with mild thyroid dysfunction. Results: Twenty-six patients with subclinical hypothyroidism and 14 patients with subclinical hyperthyroidism were included. In patients with subclinical hypothyroidism median (range) TSH was 7.8 (4.3-26.7) mU/l (reference 0.27-4.2) at diagnosis and decreased to 2.3 (0.36-4.0) mU/l following treatment with levothyroxine. Mean (+/- S.D.) CysC increased from 0.88 +/- 0.23 mg/l (reference 0.63-1.33) in the hypothyroid state to 1.01 +/- 0.21 mg/l when TSH normalized (p<0.001). In patients with subclinical hyperthyroidism, median TSH at diagnosis was 0.08 (0.001-0.26) mU/l and increased to 1.6 (0.28-4.0) mU/l in the euthyroid state. CysC declined from 1.04 +/- 0.29 mg/l at diagnosis of subclinical hyperthyroidism to 0.91 +/- 0.25 mg/l when TSH normalized (p < 0.05). Conclusions: Mild thyroid dysfunction significantly alters CysC levels. Therefore, thyroid function has to be considered when CysC is used as a marker of kidney function. (C) 2003 Elsevier B.V. All rights reserved.