To see what normal blood and urine values are, go here
For an explanation of causes of most abnormal blood and urine tests, go here
To see how tests are grouped, go here
Vitamin D, in its D3 form, is one of the fat soluble vitamins that your cat or dog obtains from its diet. You and I can do fine with dietary sources of vitamin D2 (ergosterol), its "pro" form ergocalciferol or we can simply produce vitamin D from cholesterol-like compounds in our skin (7-dehydrocholesterol) found in ingredients like linseed oil, cottonseed oil and wheat when we are exposed to unfiltered sunlight. (ref) But most carnivores, including dogs and cats, have lost that ability. Pet birds share our human ability to convert D2 to D3 - but only when they are exposed to natural sunshine unfiltered by window glass. Your dog or cat would normally obtain adequate amounts of D3 from the liver, egg, fish and other meat ingredients in its diet. Pet food manufacturers fortify their foods with extra vitamin D3 - just to make sure.
Adequate vitamin D3 is essential to maintain proper levels of calcium and phosphorus in your cat or dog's blood. That in turn, keeps your pet’s bones properly mineralized and strong. Proper vitamin D levels are particularly important in growing puppies and kittens. If not enough vitamin D3 and/or calcium is present, in their diet, the result is rickets. Proper calcium metabolism in your pet is not only dependent on its vitamin D3 stores, it also depends on normal functioning parathyroid glands and kidneys. (ref) Optimal amounts of circulating calcium is essential for normal nerve function, muscle contraction and many other vital body processes.
How much vitamin D3 your pet actually requires is very dependent on the balance between calcium and phosphorus in its diet. Things function best when there is approximately 1.2 parts calcium to 1 part phosphorus in the diet of your dog or cat. Growing puppies and kittens require more vitamin D3 in their diet than adults.
When a puppy or kitten does not receive enough vitamin D3 in its diet, the result can be twisted, bowed legs (rickets). When an adult pet does not receive enough vitamin D3, the result will be weak bones that are subject to fracture (osteomalacia, osteoporosis).
Poorly thought out, homemade diets are the most likely cause. Well-meaning pet owners might easily conclude that their pets will do just fine on the foods that they themselves do well on. That is particularly true if the owners themselves limit their consumption of meat. (ref) Vitamin D3 is not abundant in grain, vegetables, nuts or fruit - and your dog or cat can't simply cook some up with a walk in the sunshine like you and I can.
To compound that potential problem, the amount of vitamin D3 in lean red meat is quite low. Liver, egg yolks and fish oils are a much better source. Red meat has a calcium to phosphorus ratio the reverse to what dogs and cats should be consuming. That unhealthy, reversed ratio makes the effects of insufficient vitamin D3 even worse. Although no one I am aware of has published studies, during puppy and kitten development, adequate vitamin D3 intake is certainly important. (ref) As time goes by, consuming too little calcium in relation to phosphorus affects dental health as well. (ref) More recently, studies seem to indicate that adequate amounts of vitamin D3 are also important for proper immune system function. (ref)
Accidental consumption of certain rat or mouse poisons is the most common cause of excessively high vitamin D3 blood levels. That occurs more commonly in dogs than in cats. For a time, vitamin D3 rat and mouse poisons were the most common types used. Over time, that changed and poisons containing compounds that promoted fatal bleeding became the most common sold (coumarin, dicoumarol, warfarin, Coumadin®,etc.) Those were replaced at governmental insistence by rodent poisons containing bromethalin which was thought by the FDA to be less of a danger to children. However, there is no known antidote for bromethalin. So in 2019, common brands like d-CON® again contain toxic levels of vitamin D3.
The next most common cause of vitamin D3 overdose is giving your pet too much vitamin supplement or fish oils containing vitamin D (like cod liver oil).
The next most common cause is probably petfood manufacturing errors in which too much vitamin premix was added to a batch of dog or cat food. (ref)
Certain plants can perhaps also contain unsafe amounts of vitamin D3. (ref) But it is unknown if pets that accidentally ate them could end up with vitamin D overdose. Accidental consumption by a pet of certain human anti-psoriasis medications (eg Dovonex®/calcipotriol) might also be a possibility.
High blood vitamin D levels also occasionally occur in certain cancers (lymphomas) and chronic inflammations (granulomatous diseases) that stimulate macrophage/monocyte cells (eg histoplasmosis, coccidiodomycosis, blastomycosis, etc.).
Finding normal blood calcium levels in your dog or cat subsequent to its exposure to excessive amounts of vitamin D3 are not an absolute guarantee that dangerous metastatic calcification will not occur at a later date. I would repeat those tests twice over a four month period - but that is something for your veterinarian to decide.