This is what I have as well (I think). I am not aware of any of the symptoms (yet). I went to an endocrinologist 4 years ago after high levels of PTH were discovered in a blood test by my primary care provider. She sent me to the endocrinologist. My calcium levels have always been normal but my Vitamin D levels were low so she had put me on a Vitamin D3 supplement. The descriptions I read of hyperthyroidism also indicates that it can take a while for levels to return to normal if there has been a Vitamin D deficiency. This was echoed by the endocrinologist at the time - he said getting your PTH levels back to normal after a Vitamin D deficiency was like steering a ship and can be done but it takes a while to do - we have come down the road 4 years and she did another test in February. My calcium is normal but my PTH is still elevated beyond the normal range. So here I go again to an endocrinologist next week (but a different one this time). I get a bone density test on Monday morning. I read where someone else who had the same normocalcemic hyperparathyroidism made the comment that if you have a tumor (cancerous or not), that tumors can make a parathyroid gland behave erratically. It may be 100 when you get tested and the next day be 600. Not sure how true that is but it makes a certain amount of sense. There is only so much the other 3 glands can do to try to make up for the gland that may be behaving badly.

There are articles about normocalcemic hyperparathyroidism from the National Institute of Health and the UCLA Health organization here: https://www.ncbi.nlm.nih.gov/books/NBK555967/ and here: https://www.uclahealth.org/endocrine-center/normocalcemic-primary-hyperparathyroidism.

Best of luck.