Thanks for the info, everyone. Bailey's history is a complex one. She's 14 years old. The only medication she's on is lactulose. Her food contains vitamin D, but she's not on any supplements. I did think that lactulose may have been causing her elevated calcium, but after cutting her lactulose dose in half over 2 months, her calcium actually went up. I'm in the process of digging through her records in an attempt to find a pattern, but here's the abridged version of the story:
In May of 2007, elevated calcium and creatinine were noted during a trip to the emergency vet after she stopped eating. Creatinine went back to normal after getting IV fluids though. Over the next 3 months, we did just about every possible diagnostic test to find the cause. After ruling everything out, the only answer was that she had idiopathic hypercalcemia. We were about to start treating her with prednisone when her calcium levels (total and ionized) went down to almost normal level and stabilized, so it was decided not to treat it and monitor her. All of her blood work was normal (including calcium, which eventually got down to high-normal) until October of 2008, when she was diagnosed with hyperthyroidism (all other values were normal). In December of 2008, she received radioiodine treatment for hyperthyroidism. In January of 2009, follow up blood work showed normal thyroid hormone levels, and everything else was normal as well. In March of 2009, she had dental work done and pre-op blood work was all normal. In April, Bailey had a bad bout of constipation, so after getting an enema at the vet, I increased her lactulose. In August of 2009, routine blood work showed elevated total calcium (everything else was normal). I wondered if the increased lactulose may have caused this since I had previously wondered if there had been a connection back in 2007, so I decided to decrease her lactulose and have her calcium re-checked in a couple months. Now it's November of 2009, and her total calcium has gone up even more, along with the elevated creatinine.
I plan to have a complete workup done again, along with ionized calcium. Her ionized calcium level should give us an idea of where to go next. This is just confusing. All the diagnostic tests done in 2007 ruled out every known cause, so it's just really weird. I'm trying to think of anything I've done differently since January of 2009, which was the last time her calcium tested normal, and I can't think of anything. She's been on the same dry food since 2007, and she's always gotten a variety of different wet food. I guess it could be something to do with her wet food, but I'm not sure which one.
One thing I find interesting is that the first time we found her calcium to be elevated in May of 2007, she had completely stopped eating. Her total calcium was the highest it had ever been until now, yet I haven't noticed any obvious difference in her eating habits. An ionized calcium test might shed a little more light on this. I'm just at a loss, and I can't believe this is happening again.
Thanks again everyone! Hopefully I'll know more in the near future.