Walter, your veterinarian was referring to hyper and hypo THYROID, not PARATHYROID. They are both glands in the neck, but they are completely different and have different functions.
Likewise, osteoarthritis is NOT the same thing as osteoporosis. Arthritis is an inflammatory condition that destroys your joints. Osteoporosis is loss of calcium from your bones, resulting is their gradual destruction. Both can be crippling diseases, but they are different diseases with different causes.
I think Spikey made a great point. You don't need anyone's permission to take Vitamin D, and no, it's not that expensive, AND the prescription form, which is what you will get free or almost free if/when you talk someone into prescribing it is NOT the right form for someone with the DS because it's oil based. And if you know nothing else, which I'm beginning to suspect, you do at least know that oil is fat and we don't absorb much fat, right? So oil based vitamin D won't do the trick, you need the DRY form from Vitalady that you will have to pay for yourself. And it's not terribly expensive. Like you, I spend far less on vitamins and meds for myself than for my pets.
So again, stop waiting to find just the right doctor who actually understands the DS and think for yourself. Heck, we've done all the thinking for you! All you have to do is get the dry D and start taking it in the right amounts. Is that really such a strain?