BOOGEE,
I think it is good to check it once in a while. Vitamin D helps with absorption of calcium and is important for overall bone health, especially for somone on HT. HT will reduce bone density. If your Dr checks the vitamin D, they will provide a range to go with it. I assume they are monitoring his bone density and advising him on supplements and possibly using Zometa, XGEVA, or Prolia if needed. I am on Prolia -- every 6 month injections.