My husband had parathyroidectomy four months back. everything went fine post procedure. lately his pth is again high. his calcium level is normal.

Check Vitamin D: Low Vitamin D level ( 25 OH Vit .D) is a common cause of high PTH level while Calcium level is Normal. A condition called Secondary Hyperparathyroidism. Check 25 OH Vitamin D & Ionized Calcium, If ionized Calcium is NL & Vitamin D is less than 30 start Vitamin d treatment and that should correct the PTH level.

I agree vitamin d: Vitamin D deficiency is the most likely explanation. The second most likely explanation is a failed surgery due to unrecognized double tumor or 4 gland disease. You should check calcium for a while, if it returns high, the HPT is not treated. In the meantime, a 25 OH vit D level should be drawn and treated appropriately.