Hyperthyroidism is characterized by increased bone turnover and resorptive activity. Raised levels of serum osteoporotic cytokines, such as interleukin (IL)-1beta, IL-6 and tumor necrosis factor (TNF)-alpha have been demonstrated previously in hyperthyroidism. These elevations are controversial and it is difficult to differentiate the contribution of thyroid hormones to the elevation of cytokines from that of the autoimmune inflammation in Graves' disease (GD) and follicular cell damage in thyroiditis.. Therefore, we investigated the effect of thyroid hormones on serum IL-1beta, IL-6, TNF-alpha levels and bone metabolism on L-thyroxine induced hyperthyroid rats and changes in cytokine levels and bone metabolism on the same rats after reversal to euthyroidism. Rats were treated with L-thyroxine for 5 weeks (0.4 mg/ 100 g food). Plasma T-3, T-4, TSH and serum IL-1beta, IL-6, TNFalpha, Calcium (Ca), phosphorous (P), parathyroid hormone (PTH), alkaline phosphatase (ALP), bone alkaline phosphatase (B-ALP) levels were measured and differential leucocyte counts were made initially, at the 5th week of the experiment (hyperthyroid state) and 5 weeks after quitting the administration of L-thyroxine (euthyroid state). Significant rises in serum IL-1beta, IL-6 and TNFalpha were noted in hyperthyroidism (P<0.001). In euthyroid state, IL-1beta, IL-6 and TNFalpha decreased significantly, but IL-beta and TNFalpha were significantly higher than the baseline values (P<0.05) while IL-6 levels turned back to the baseline values. Plasma T3 and T4 levels were significantly correlated with serum cytokines in hyperthyroid state while there was no correlation in euthyroid states. Ca and P levels did not differ significantly while PTH levels declined significantly in the hyperthyroid state (P<0.05). After the reversal to the euthyroidism, there was no significant change in Ca, P and PTH levels. ALP and B-ALP increased significantly in hyperthyroidism (P< 0.001, P<0.01) and they did not decrease in euthyroid state. The lymphocyte number and ratio in differentials increased significantly in the hyperthyroid state (P<0.001). In euthyroidism they decreased significantly (P<0.001) but it was significantly higher than the baseline value (P<0.05). Our findings showed that the deleterious effect on bone metabolism in hyperthyroidism might be mediated by cytokines and the increased bone turnover in hyperthyroidism failed to decrease despite euthyroidism.