Background: We have previously found decreased serum levels of both ionized calcium and 1,25(OH)<sub>2</sub>D and an increase in serum phosphate levels at 1 year after hemithyroidectomy. However, basal and stimulated parathyroid hormone (PTH) secretions were not altered. To investigate whether the observed biochemical changes after unilateral thyroid surgery may be due to a relative end-organ resistance to PTH, we studied the peripheral effects of infused hPTH-(1–34) in 6 patients preoperatively and 3 months after hemithyroidectomy. Methods: Serum levels of TSH, FT<sub>4</sub> and FT<sub>3</sub> were measured pre- and postoperatively. hPTH-(1–34) was infused at 0.9 IU/kg/h during 6 h. Blood samples for analysis of ionized calcium, intact PTH, phosphate, 25(OH)D, 1,25(OH)<sub>2</sub>D and urinary samples for calcium, phosphate and nephrogenous(n)-cAMP analysis were taken at baseline, when the infusion was discontinued after 6 h and at 24 h. Results: Three months after hemithyroidectomy, serum levels of FT<sub>3</sub> were decreased and TSH levels increased. Pre- and postoperative hPTH-(1–34) infusions induced increased serum levels of ionized calcium, 1,25(OH)<sub>2</sub>D, increased urinary excretion of phosphate and elevated n-cAMP levels. The changes in the studied biochemical variables during the hPTH-(1–34) infusions did not differ between the two study occasions. Conclusion: By using a 6-hour hPTH-(1–34) infusion protocol, we have shown that the peripheral PTH effect is not altered by a slight reduction in thyroid hormone levels at 3 months after hemithyroidectomy.