Hyp mice, the murine homolog of human X-linked hypophosphatemia (XLH), are characterized in part by elevated renal loss of phosphate (Pi) and hypophosphatemia. The lesion responsible for this syndrome is believed to be defective Na+ gradient-dependent phosphate transport across renal brush border membranes (BBM). It has been demonstrated that thyroid hormones can stimulate Pi uptake by rat BBM vesicles (BBMV). The purpose of this current study was to determine whether thyroid hormones could increase renal BBMV transport of Pi and improve the renal conservation of Pi in Hyp mice. Hyp mice were treated with either vehicle (controls) or T4 (0.2 mg/100 g BW, ip, once daily) for 6 days. At the end of the treatment period, the uptake of 32Pi (at 30 sec), in the presence of a Na+ gradient (Na+o greater than Na+i) by BBMV prepared from renal cortex of T4-treated Hyp mice, was significantly increased (+25%) compared to that in vehicle-treated Hyp controls. The uptake of L-[3H]proline (at 10 sec) by the same BBMV preparation was unchanged. T4-treated Hyp mice displayed a significant decrease in urinary Pi excretion (-31%), and a restoration of plasma Pi levels (+52%) to the normal range. Plasma calcium, creatinine, and magnesium as well as urinary excretion of cAMP did not change with T4 treatment. Another group of Hyp mice was given T3 (0.2 mg/100 g BW, ip) at three 12-h intervals. After 36 h, the rate of 32Pi uptake by renal BBM was significantly increased (+40%). Plasma Pi also increased (+26%). Our findings indicate that parenteral administration of thyroid hormones specifically increased (compared to transport of L-proline) the capacity of Na+ gradient-dependent renal BBM Pi uptake in Hyp mice, decreased renal Pi excretion, and increased plasma Pi. These results suggest that treatment with T4 or T3 can enhance the defective proximal tubular reabsorption of Pi and improve renal Pi retention in murine X-linked hypophosphatemia.