Background: Previously we have found reduced levels of total serum calcium and 1,25(OH)<sub>2</sub>D<sub>3</sub> despite an unaltered stimulated parathyroid hormone (PTH) secretion 1 year after hemithyroidectomy. The present study was undertaken to elucidate the possible relationship between calcium homeostasis, thyroid hormones and bone resorption in a group of 45 consecutive patients subjected to hemithyroidectomy because of a solitary nodule. All patients had free T<sub>4</sub> and T<sub>3</sub> levels within normal range preoperatively. Methods: Thyroid hormones, bone mineral and biochemical variables known to reflect calcium homeostasis were studied. Patients were divided into three separate groups depending on their pre- and postoperative thyroid hormone status. Results: One year postoperatively, serum levels of free T<sub>4</sub> were decreased and that of thyrotropin (TSH) increased in the entire group of patients. The concentration of ionized calcium was reduced from 1.25 ± 0.05 to 1.22 ± 0.04 (p < 0.001) despite an unaltered PTH value (2.8 ± 1.0 vs. 3.1 ± 1.5, p = 0.50). A significant reduction in C-terminal telopeptide type 1 collagen (1CTP) indicated decreased bone resorption 1 year after surgery (p < 0.05). Subgroup analysis showed that a reduction in ionized calcium was seen only among patients with a postoperative decrease in free T<sub>4</sub>. Patients with subclinical hyperthyroidism preoperatively presented the lowest postoperative levels of ionized calcium, significantly reduced levels of 1CTP and increased levels of phosphate and creatinine. Multiple linear regression analysis showed that age (p < 0.05) and postoperatively changed serum levels of TSH (p < 0.05), creatinine (p < 0.05), phosphate (p < 0.001) and FT<sub>4</sub> (p < 0.01) were independently associated with altered levels of ionized calcium. Conclusion: We conclude that the reduction in ionized calcium 1 year after hemithyroidectomy was not due to PTH deficiency. Instead our results suggest that the reduced effects of thyroid hormones on bone and kidney function is essential.