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