From Dr. James Norman of the Norman Parathyroid Clinic:
Normal blood levels of parathyroid hormone vary according to the lab that measures the hormone, but most labs have a normal level between 15 and 65. Some patients have a NORMAL parathyroid level but do have parathyroid disease. The most patients with primary hyperparathyroidism (due to a benign tumor in their neck) have PTH levels ranging from 60 to 150. When patients have PTH levels above 200 it is because their tumor is very old (usually this means over 10-12 years old) and the tumor is composed of MANY cells. If your PTH level is over 200, the amount of destruction to your body is generally worse. Also note that about 15% of patients that have a parathyroid tumor in their neck will have PTH levels that are in the "normal range". Let me say that again... about 15% of patients with hyperparathyroidism will have high calcium levels and NORMAL PTH levels. Realize that this is still hyperparathyroidism and they still have a parathyroid tumor in their neck... We say their PTH level is "inappropriately normal". In other words, it still shows that there is a tumor full of crazy, out of control cells because if they were normal parathyroid cells... they would sense the high calcium and they would shut down. If your calcium is high, and your PTH is not near zero (consistently 20 or below), then you almost certainly have a parathyroid tumor. GET IT OUT!