Low phosphorus levels (blood work)

Has anybody had a dog with low phosphorus levels? Haley's level was low in her recent blood work. It dropped significantly from her last blood work in July. It went from 3.1 down to 1.8 (normal range listed at 2.5-6.0). I have found different analysis online stating it could be from overactive thyroid, different cancers, malnutrition, malnutrition, and others. Her potassium is getting near the lower level in the normal range (not below yet). Her Free T4 (thyroid) is 32 with the normal range being 8-40, so it is getting closer to the upper range. That would make me think the low phosphorous could possibly be from overactive thyroid (if so, bad news, because most likely thyroid cancer for overactive), but her regular T4 is not high. It is 2.4 (normal range 1.0-4.0). There were a couple other items (total protein and albumin) that are in the normal range, but towards the upper end. I read that dehydration could cause both of the those to be high (among a couple other reasons), so that could explain those -- not sure if she even drinks any water during the day while I'm gone at work, and her blood test was done after work.

I guess my main issue is with the low phosphorus. Oh I hope it's not from hyperthyroidism. My last dog had HYPOthyroidism, which was very easy to control with bovine thyroid extract. One thing, though, is that Haley is not underweight (not even close -- she's actually a little over what she should probably be).

Thanks for ANY help that can be offerred!!!
Mindy