Interleukin-6 (IL-6) has been shown to be involved in the pathogenesis of several bone diseases characterized by a negative balance between bone resorption and formation. The aim of the study was to estimate serum markers of bone turnover: osteoclast-derived tartrate-resistant acid phosphatase form 5a (TRACP 5b) reflecting resorption, and osteocalcin as a marker of bone formation in IL-6 knock-out mice to assess the role of IL-6 in the pathogenesis of thyrotoxicosis-related disturbances of bone metabolism. The study was performed on forty, 14-15 weeks old, female mice: C57BL/6J (wild-type; WT) and C57BL/6J (IL6-/-Kopf) (IL-6 knock-out; IL6KO). The mice were randomly divided into 4 groups with 10 mice in each one: 1. WT mice in thyrotoxicosis (WT-thx), 2. WT controls (WT-ctrl), 3. IL6KO mice with thyrotoxicosis (IL6KO-thx), and 4. IL6KO controls. Experimental model of hyperthyroidism was induced by intraperitoneal injection of levothyroxine at a dose of 1 microg/g, daily over 21 days. The serum levels of TRACP 5b and osteocalcin were determined by ELISA. Serum concentration of TRACP 5b (median and interquartile ranges) were significantly increased in both groups of mice with thyrotoxicosis: WT [28.2(18.8-41.6) U/l] and IL6KO [26.4(23.0-31.2) U/l] as compared to the respective controls. Osteocalcin serum levels in IL6KO-thx mice [111.9(103.1-175.6) ng/ml] were significantly elevated in comparison to WT-thx animals [46.1(32.5-58.9) ng/ml]. In summary, the results of the present study suggest that IL-6 plays a crucial role in thyrotoxicosis-related disturbances of bone turnover in mice, determining the imbalance between bone resorption and bone formation caused by excess of thyroid hormones predominantly by inhibition of bone formation.