To interpret this, we need more information. A PTH of 90 is above the “normal” range, but we cannot yet give a diagnosis because we need the calcium level. PTH should only be interpreted in the context of calcium levels.

The parathyroid glands make PTH in order to regulate calcium levels. If the blood calcium level is low, then the parathyroids will make more PTH in order to get the calcium up. This is a normal response, and indicates that the parathyroid glands are doing what they are supposed to be doing. So if you told me that your calcium has been around 8.5 mg/dl (or 2.12 mmol/l), then I would say that a PTH of 90 is perfectly appropriate. That calcium level is low, so the parathyroids should be making more PTH than normal in order to try to get it up. (For whatever reason, they may not able to get the calcium level up.) The treatment for this is to increase calcium and Vitamin D supplementation, so the parathyroid glands don’t have to work so hard. Once the calcium is in normal range, the PTH level should drop back to normal. 

On the other hand, if your calcium level is 10.8 mg/dl (2.62 mmol/l), then a PTH of 90 indicates primary hyperparathyroidism, usually caused by a parathyroid tumor. This calcium level is high, and the parathyroid glands should sense this and “turn off” - meaning stop making more PTH. If the glands continue to make a lot of PTH even when the calcium is high, that indicates a problem with one or more of the parathyroids (primary hyperparathyroidism). The treatment for this is parathyroidectomy, or surgery to remove the diseased parathryoid gland or glands.