We know that overactive parathyroids (hyperparathyroidism) is a side effect of treatment (and possibly of the underlying biochemistry of XLH even without treatment), but we don't know the extent to which it exists or what treatment is generally used.
Sometimes adjusting or discontinuing treatment can reduce the parathyroid hormone (PTH) levels, and other times Sensipar (cinacalcet) will reduce PTH levels, and sometimes neither treatment works. Some patients undero either partial or complete parathyroidectomy (removal of the parathyroid glands), but it's complicated surgery and if the parathyroidectomy is partial, we've heard of cases where the glands regenerate and become enlarged again.
Have you had overactive parathyroid glands, and if so, what treatment did you have and was it successful?